So, this sends an invoice after your customers paid? Isn't it normal that you send an invoice which the customer then pays?
A lot of e-merchants I order at (even professionally) require immediate payment during checkout, and send the invoice only after the order is shipped.
Back on topic: I rather like these "simple" solutions, but the pricing seems to high for my taste, if you think of it: they are just generating and mailing PDF's. Also, invoices usually have requirements that are different per country, so I doubt their solution is universal enough.
Invoices are equal in all the EU countries: number, date, customer data, vendor data, items, VAT... And Quaderno supports multi-currencies, multi-languages, and multi-taxes :)
Tax invoices are still invoices: issued before payment.