It is important to have a contact who will make sure that invoices are approved and get paid in a reasonable time. If not they have a tendency to go missing.

It is important to have a contact who will make sure that invoices are approved and get paid in a reasonable time. If not they have a tendency to go missing. The important thing is tracking what has been authorized for payment. Without solving that a protocol would risk being just another black hole where unpaid invoices languish. Instead build something that tracks approved invoices. When an invoice is approved the vendor gets an email asking them to fill in a form, entering all the data from the invoice. Include a big "pay me" button at the bottom. Don't make the payment until the data is provided.