Invoices generated right now are useless.
Proper invoces must have:
* Issuer information (Company/organization, address, vat number, etc...) In this case I think the issuer is the Drupal Association.
* Invoice number (usually every invoice a company issues have a serial number: 1, 2, 3 ,4...).
Invoices should be generated as soon as posible, so the buyer can use it for accounting and taxes.
I'd would be nice to have them in PDF.