Fetchflow.com is a rapidly growing invoicing app that provides a full set of free services. You can work with their solid API and get most of your invoicing tasks done with ease. Here's the API: http://www.fetchflow.com/API/
You should try fetchflow. They offer full featured invoicing solution for free. It is OS independent as it is web based. You can create Unlimited PDF [Adobe PDF] branded invoices, estimates & PO's. There are many cool features like paypal integrated invoice payments,MS-Excel template based invoicing,Integrated stock & catalog invoicing etc.And many useful add-on,s are also available.