Here in Norway, electronic invoices are spreading, and the solution promoted by the Norwegian government require that invoices are sent through one of the approved facilitators, and it is not possible to send electronic invoices without an agreement with one of these facilitators. This seem like a needless limitation to be able to transfer invoice information between buyers and sellers. My preferred solution would be to just transfer the invoice information directly between seller and buyer, for example using SMTP, or some HTTP based protocol like REST or SOAP. But this might also be overkill, as the "electronic" information can be transferred using paper invoices too, using a simple bar code. My bar code encoding of choice would be QR codes, as this encoding can be read by any smart phone out there. The content of the code could be anything, but I would go with the vCard format, as it too is supported by a lot of computer equipment these days.
The vCard format support extentions, and the invoice specific information can be included using such extentions. For example an invoice from SLX Debian Labs (picked because we ask for donations to the Debian Edu project and thus have bank account information publicly available) for NOK 1000.00 could have these extra fields:
X-INVOICE-NUMBER:1 X-INVOICE-AMOUNT:NOK1000.00 X-INVOICE-KID:123412341234 X-INVOICE-MSG:Donation to Debian Edu X-BANK-ACCOUNT-NUMBER:16040884339 X-BANK-IBAN-NUMBER:NO8516040884339 X-BANK-SWIFT-NUMBER:DNBANOKKXXX
The X-BANK-ACCOUNT-NUMBER field was proposed in a stackoverflow answer regarding how to put bank account information into a vCard. For payments in Norway, either X-INVOICE-KID (payment ID) or X-INVOICE-MSG could be used to pass on information to the seller when paying the invoice.
The complete vCard could look like this:
BEGIN:VCARD VERSION:2.1 ORG:SLX Debian Labs Foundation ADR;WORK:;;Gunnar Schjelderups vei 29D;OSLO;;0485;Norway URL;WORK:http://www.linuxiskolen.no/slxdebianlabs/ EMAIL;PREF;INTERNET:firstname.lastname@example.org REV:20130212T095000Z X-INVOICE-NUMBER:1 X-INVOICE-AMOUNT:NOK1000.00 X-INVOICE-MSG:Donation to Debian Edu X-BANK-ACCOUNT-NUMBER:16040884339 X-BANK-IBAN-NUMBER:NO8516040884339 X-BANK-SWIFT-NUMBER:DNBANOKKXXX END:VCARD
The resulting QR code created using qrencode would look like this, and should be readable (and thus checkable) by any smart phone, or for example the zbar bar code reader and feed right into the approval and accounting system.
The extension fields will most likely not show up in any normal vCard reader, so those parts would have to go directly into a system handling invoices. I am a bit unsure how vCards without name parts are handled, but a simple test indicate that this work just fine.
Update 2013-02-12 11:30: Added KID to the proposal based on feedback from Sturle Sunde.