Belgium's structured B2B e-invoicing mandate changes what "issuing an invoice" means. A PDF emailed to your customer is no longer the invoice for transactions in scope — the legal document is a structured UBL file exchanged over the Peppol network, addressed to the buyer's registered participant identifier, and matched against a schema plus the EN 16931 business rules and the Belgian CIUS. Finance teams running ERPNext today submit a Sales Invoice, print the standard format and email it. Nothing in that flow produces a Peppol BIS Billing 3.0 payload, resolves the customer's Peppol ID, talks to an access point, or records the transport acknowledgement that proves you delivered on time.

ERPNext core gives you an excellent accounting spine — Sales Invoice, Item Tax Templates, Tax Categories, Company and Customer address masters, the GL — but it stops at the network boundary. There is no Peppol participant lookup, no UBL serialisation of your tax lines, no place to hold a message-level rejection from the receiver so an AR clerk can act on it, and no immutable archive of the exact bytes that were transmitted. Teams fill that gap with a spreadsheet and a provider portal, which means the ERP and the legal record drift apart within a month, and month-end reconciliation becomes an exercise in trusting two systems that disagree.

What ECOSIRE builds is a proper Frappe app — its own module, DocTypes, hooks and fixtures — installed onto your bench alongside ERPNext, never a pile of patches inside erpnext. A Peppol Settings single DocType holds your access point endpoint, credentials (stored as password fields, not plain text), your own participant identifier scheme and value, and the switch between test and production. A Peppol Document DocType is created on Sales Invoice on_submit via a hooks.py doc event; it carries a link back to the invoice, the generated UBL as a file attachment, transmission state, the access point message id, and a child table of validation findings. Serialisation maps ERPNext master data onto EN 16931 semantics: Company and Customer addresses become AccountingSupplierParty and AccountingCustomerParty with the correct endpoint IDs, Item Tax Templates and Tax Categories resolve to Belgian VAT category codes and exemption reason texts, payment terms become PaymentMeans and PaymentTerms, and UOMs map to UN/ECE Rec 20. The same posting drives both outputs — the machine-readable UBL and the human-readable Print Format — so the copy your customer reads and the file the authority sees can never disagree.

Transport and recovery are where a build-to-order app earns its cost. A scheduler event polls your access point for message-level and business-level responses and writes each one back onto the Peppol Document, moving it through queued, sent, acknowledged or rejected. A rejection is not a dead end: the app records the reported rule id and message, blocks silent re-sends, and drives a correction workflow that guides the AR user to a Credit Note or an amended invoice, then links the replacement to the original so the audit chain stays intact. Whitelisted methods expose the same actions over the Frappe REST API for your own automation, permissions run through a dedicated role and role profile so only authorised finance users can transmit or retry, and every transmitted payload plus every response is retained as an attachment on the submitted invoice — the archive and audit trail an inspection actually asks for.

This is a build-to-order engagement, not a download. We start with a scoping call to confirm your ERPNext version (v15 or v16), your bench and hosting model, your chosen access point provider, the invoice types in scope, and your VAT edge cases — intra-community supplies, reverse charge, exemptions. We then build against your configuration, deliver onto a staging site for UAT with your real invoice shapes, and go live behind a documented rollback plan. Typical delivery is one working week from confirmed scope. You receive the full source repository, so you own the code and can maintain it yourself if you ever choose to.