Our API directory now includes 35 invoicing APIs. The newest is the CashFlows API. The most popular, in terms of mashups, is the PayPal API. We list 33 PayPal mashups. Below you'll find some more stats from the directory, including the entire list of invoicing APIs.
PayPal has announced the release of a new PayPal Invoicing API, which aims to provide small businesses with the tools necessary to easily create, send, and process invoices. Although targeting small businesses, the company has noted that the API has a global reach.