Where I live (the Netherlands), a receipt is usually just a small slip of paper you receive in a supermarket of gas station (as a private person). An invoice is usually only send to businesses and are more comprehensive than receipts.

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.