Mercurius and B2G e-invoicing: invoicing Belgian public bodies

To invoice a Belgian public body (a commune, a CPAS, a region, a federal service), you send a structured e-invoice over Peppol to the authority's enterprise number and quote the order number it gave you. Mercurius, run by FPS BOSA (the Federal Public Service Policy and Support), is the public sector's mailroom that receives the invoice on the authority's side. You never send "to Mercurius", and you need no Mercurius account if your software sends Peppol.

This guide is for suppliers to the public sector and their accountants: when the B2G rule applies, how to find the authority's Peppol ID and reference, how to send and follow the invoice in OrdoGrid, and how to fix a rejection. The VAT obligation between businesses is covered in the Belgian B2B e-invoicing obligation, order numbers for all customers in purchase order numbers on invoices.

B2G and B2B: two rules on the same network

In both cases the recipient's Peppol ID is 0208: followed by its enterprise number. Selling mainly to public bodies does not exempt you from the B2B rule: since 1 January 2026, a business established in Belgium with a VAT number must be able to issue and receive Peppol BIS invoices (art. 13quater Royal Decree no. 1), for instance from its own suppliers. Not having the technical means is punishable by fines of €1,500, then €3,000, then €5,000 (Royal Decree no. 44). A Mercurius portal login does not make you reachable for those invoices.

When an e-invoice to a public buyer is compulsory

What the law requires. Directive 2014/55/EU obliges contracting authorities to accept e-invoices that follow the European standard EN 16931. Belgium went further: art. 14/1 of the Law of 17 June 2016 on public contracts obliges suppliers to send their invoices electronically to all contracting authorities, federal, regional and local. The Royal Decree of 9 March 2022 set the start dates by contract value and the €3,000 excl. VAT exemption; the Royal Decree of 4 September 2023 postponed the last phase.

An electronic invoice means a structured invoice the buyer can process automatically. The Brussels Region's supplier guide states that a PDF, Excel or XML file sent by email is not one and may be refused.

Contracting authority

A buyer subject to public procurement law (pouvoir adjudicateur, aanbestedende overheid). FPS BOSA's FAQ says to check the KBO/BCE public search: if the section "Capacities" shows "Contracting authority – Since [date]", the B2G obligation applies.

What Mercurius is, and what it is not

Mercurius

The central e-invoicing and e-procurement mailroom of the Belgian public sector, run by FPS BOSA. It receives invoices, credit notes and attachments over Peppol on behalf of the contracting authorities connected to it, passes them to their systems and returns their responses.

Portal access. You log in with your eID or itsme. Legal representatives registered in the KBO/BCE get access automatically; anyone else, such as an invoicing clerk, first needs the role "invoicing manager" through the eGov role management application (BOSA Mercurius FAQ). A portal account does not register you on Peppol.

Because you address the authority's own Peppol ID, it does not matter whether it receives through Mercurius, its successor or another provider. What matters is that the ID is registered when you send.

Find the authority's Peppol ID and the reference it checks

1. Identify the right legal entity. Take the name on the order form, not the town: a commune and its CPAS are separate entities with their own enterprise numbers. In the KBO/BCE public search, note the ten-digit enterprise number, the "Contracting authority" capacity and whether a VAT number is active.
2. Check that it receives on Peppol. Enter 0208: followed by the ten digits on lookup.peppol.org, as FPS Finance (the federal tax administration) recommends. Belgian participants must be registered at least under their enterprise number, so you need not ask for a "Peppol ID". See look up a Peppol participant.
3. Get the reference the authority checks. Authorities match invoices to their order form; BOSA's FAQ names a wrong order number as a usual reason for rejection. If the order form is unclear, ask the purchasing contact which number to quote.

Peppol rule PEPPOL-EN16931-R003 rejects an invoice or credit note without a buyer reference or an order reference. OrdoGrid always fills the buyer reference, so this rule never blocks you. If an authority insists on its order number in BT-13, OrdoGrid cannot fill that field: tell the authority it is in the buyer reference, or use the portal for that buyer.

Send a B2G invoice from OrdoGrid

OrdoGrid has no Mercurius-specific feature: a public buyer registered on Peppol is a recipient like any other. You need a company with a BE VAT number and a signed Autorisation Peppol; see set up Peppol in OrdoGrid.

1. Create the client. Open Clients in the sidebar (page Base Clients), click Ajouter un client (Add a client), choose Assujetti à la TVA (VAT-registered business, the only client type with a number field) and type the number in N° de TVA. The lookup fills in name and address from Registre belge (BCE).
2. Require the order number. Tick Exiger un bon de commande pour les factures (Require a purchase order for invoices). Format dans les notes (default PO: {input}) sets how the number is copied into the notes. Save with Créer le client.
3. Draft the invoice. Open Invoices (page Factures), click Nouvelle facture (New invoice), select the client and enter the lines with Quantité, Prix Unitaire and TVA. Set Échéance (due date) to the contract's payment term; the default is 30 days.
4. Enter the order number. In the sidebar section Documents et Références, fill N° bon de commande; OrdoGrid will not save the invoice without it for this client. Attach any document the contract asks for, keeping the total under 2 MB and preferring PDF, PNG, JPEG, CSV or XLSX, which Peppol accepts.
5. Send over Peppol. Click Envoyer. Envoi électronique via Peppol (electronic sending via Peppol) is ticked when the authority is found on the network. We recommend unticking Email unless the authority asked for a copy: the Belgian Peppol service-provider forum advises against sending the same invoice twice. Click Sauvegarder & Envoyer (Save & Send). Details: send your first Peppol invoice.
6. Follow delivery. Open Peppol → Outbox. Traité means delivered over the network; Vérifier (check) refreshes one document's status.

Once an invoice is sent over Peppol, its amounts and lines are locked. Corrections go through a credit note.

Worked example: furniture for a municipality

Atelier Nord SRL, Namur (enterprise number 0123.456.749), won a contract to supply furniture to the Commune de Valdor (enterprise number 0987.654.394); both are fictional. The contract was published in April 2026 with an estimated value of €18,000 excl. VAT, so e-invoicing is compulsory. On 2 October 2026 the commune sends order form COM-2026-441; delivery follows on 9 October.

Checks. The KBO/BCE shows "Contracting authority – Since [date]" and an active VAT number (not every authority has one; see the edge cases). On lookup.peppol.org, 0208:0987654394 receives Peppol BIS invoices and credit notes.

Invoice F-2026-0187, dated 12 October 2026, due 11 November 2026:

Total including VAT: €12,100.00. N° bon de commande holds COM-2026-441, so the XML carries it as buyer reference and the notes read "PO: COM-2026-441". Atelier Nord unticks Email, clicks Sauvegarder & Envoyer, and that afternoon the Outbox shows Traité.

Correction. On 20 October the commune reports one damaged chair. Atelier Nord selects F-2026-0187 in Factures, clicks Note de crédit and reduces the lines to one chair:

Credit notes have no order-number field, so Atelier Nord adds "PO: COM-2026-441" to Notes. Because the credit note refunds Belgian VAT to a customer with a VAT number, OrdoGrid itself adds the statement "T.V.A. à reverser à l'Etat dans la mesure où elle a été initialement déduite" (VAT to be repaid to the State insofar as it was initially deducted) in the document language, on the PDF and in the XML; Atelier Nord checks it in Aperçu du PDF (PDF preview). Royal Decree no. 4 (art. 4 §1) requires this statement on the copy sent to the customer in the situations of art. 79 VAT Code, for the supplier to recover the VAT; a credit like this one normally qualifies, but confirm with your accountant (see correct or credit an invoice). The XML carries that statement and the Notes in its document note (BT-22), so the order number reaches the authority only as free text. The credit note's XML carries its own number NC-2026-0009 as buyer reference (credit notes have no payment communication either), and its only structured link to the order is the billing reference to F-2026-0187. Before the first credit note, ask the authority whether it accepts this. Outstanding: €12,100.00 − €193.60 = €11,906.40 (net €9,840.00 + VAT €2,066.40). When the commune pays on 6 November, Atelier Nord records €11,906.40 with Record payment and the invoice shows Payée (paid). Atelier Nord keeps both documents until at least 31 December 2033.

Track the invoice: OrdoGrid and Mercurius statuses

Invoice Responses reach the provider that holds your Peppol registration: with reception active in OrdoGrid they appear on the invoice; in Envoi seul (send-only) mode they go to the provider that holds your registration, and if none does, they cannot reach you (and you do not meet the B2B receiving duty either).

Rejections and other problems

Full procedure: fix a rejected Peppol invoice.

Edge cases and common mistakes

Authority without a VAT number. Non-taxable public bodies generally have none, and BOSA's guidance is then to leave the buyer's VAT number out of the invoice. OrdoGrid has no enterprise-number field: it finds the recipient from N° de TVA, prints that number after "TVA:" on the PDF and sends it as the buyer's VAT identifier. OrdoGrid therefore cannot send a compliant Peppol invoice to an authority without a VAT number. Do not enter its enterprise number with a BE prefix in N° de TVA: OrdoGrid would send and print it as the buyer's VAT number, an element BOSA says must be left out. For such a buyer, use the Mercurius portal while it remains available, or another Peppol tool that can address the authority's 0208 ID without a VAT number.

Small or older contracts. Outside the decree, the contract documents can still require e-invoices. Read them before sending a paper or PDF invoice.

Building works. The construction reverse charge (art. 20 Royal Decree no. 1) applies only when the customer files periodic VAT returns; otherwise you charge VAT normally. Do not accept an OrdoGrid suggestion of Régime cocontractant unless the conditions are met; check with your accountant.

Another identifier. Some entities also use a GLN. OrdoGrid only reaches the 0208 (enterprise number) or 9925 (BE VAT number) identifiers derived from the client's number. Ask the authority to confirm its 0208 registration, which Belgian rules require of every participant.

VAT-registered authority buying for a taxable activity. The B2B rule may apply as well; FPS Finance has published no specific guidance, but both rules lead to the same Peppol invoice.

Credit notes to an authority. OrdoGrid credit notes have no order-number field and no payment communication. An order number you add to Notes reaches the authority on the PDF and in the XML's document note (BT-22), but only as free text: the XML carries the credit note's own number as buyer reference, and the original invoice number as billing reference. Before your first credit note to an authority, ask whether that is enough to match it to the order.

Suppliers without a Belgian VAT number. OrdoGrid sends Peppol only for companies with a BE VAT number. The Mercurius portal requires registration in the KBO/BCE and a Belgian eID or itsme login, so a foreign supplier sends through a Peppol access point that serves it (BOSA Mercurius FAQ).

Evidence to keep

Keep invoices and credit notes for seven years from 1 January of the year after issue (art. 60 VAT Code, as restored by the Law of 18 December 2025). Mercurius is a mailroom, not your archive, and OrdoGrid gives no retention guarantee, so export your own copies: see invoice archiving.

FAQ

What is the Mercurius platform?

The central mailroom of the Belgian public sector, run by FPS BOSA. It receives e-invoices over Peppol for contracting authorities and passes them to their accounting systems. The current service stops in December 2026 and moves to a new platform.

Do I need a Mercurius account to invoice a commune?

No, if your software sends Peppol: you send to the commune's enterprise number and Mercurius receives it on the commune's side. An account is only needed to enter invoices manually or look up their status.

Is Mercurius my Peppol access point?

No. Your access point, reached through your invoicing software, sends your invoices; Mercurius receives for the authorities. A Mercurius account does not let you receive invoices from your own suppliers.

Is a PDF by email enough for a public buyer?

Not for contracts covered by the B2G rule: a PDF is not a structured electronic invoice and may be refused. For contracts under the exemption, follow the contract documents.

How do I find a public body's Peppol ID?

Find its enterprise number in the KBO/BCE and enter 0208: followed by the ten digits on lookup.peppol.org. Check that it is the entity named on the order, for example the CPAS rather than the commune.

Where do I put the order number in OrdoGrid?

In the invoice sidebar under Documents et Références, field N° bon de commande, which appears once the client requires a purchase order. OrdoGrid sends it as the buyer reference.

My invoice shows Traité but the authority has not paid. What now?

Traité only means delivered. Check the Mercurius portal if you have access, or ask the authority's accounting department with the invoice number and delivery date. Do not send the invoice again.

Does invoicing public bodies over Peppol cover my B2B obligation?

Only the sending side. Since 1 January 2026 you must also be able to receive structured invoices from your Belgian suppliers: activate reception in OrdoGrid or keep it with another provider.