Cross-Platform Mobile App Build

Hello,
I am looking for an experienced mobile developer (Flutter, React Native, or native iOS/Android) to build a Minimum Viable Product (MVP) for an invoicing app. The key requirement is that invoices must be Peppol BIS 3.0 compliant (EN16931 standard) and sent through the Peppol network via an Access Point API.
Scope of work (MVP):
1. Invoice creation (UI):
o Seller, buyer, invoice number/date, currency
o Line items with quantity, unit price, VAT
o Totals (net, VAT, gross)
2. UBL generation:
o Map invoice fields to UBL 2.1 XML according to Peppol BIS 3.0
o Validate against XSD and Schematron rules
3. Peppol network integration:
o Connect to an Access Point provider’s API (e.g. Storecove, Bizbrains, Qvalia)
o Send invoices through the Peppol network
o Handle delivery receipts / error messages
4. Customer lookup:
o Query the Peppol directory (via API) to retrieve buyer participant IDs
5. Testing & validation:
o Use test recipients and official BIS 3.0 validation tools
Optional (Phase 2, not included in MVP):
• Export invoice to PDF
• Archive invoices (drafts & sent)
• Multi-language support (EN/FR/NL)
• Receiving invoices (Peppol inbox)
Deliverables:
• Mobile app (iOS + Android or cross-platform)
• Backend (if required for validation or API calls)
• Documentation on how to generate and send Peppol-compliant invoices
Skills required:
• Mobile app development (Flutter / React Native / iOS / Android)
• XML & UBL familiarity
• Experience with APIs (REST/JSON)
• Knowledge of Peppol BIS 3.0 / EN16931 is a strong plus