invoicefu is not an application with an API added as an afterthought. It's been designed to be used as a service from the first moment.
We identified the lack of a reliable and simple online invoicing service and we built it; so developers like you can use our service directly from their web or mobile applications. Then we decided to build an application for end users around that service.
The first component we developed was our service API, and the rest of the product grew from there.
Developers and integrators are our number one priority.
APIs are the interface we have with developers, but traditionally much more effort is put on UX and visual design than on API design. As developers, we have tried to carefully curate a convenient no-nonsense API.
We have built a self-discoverable REST API. It's versioned, consistent, and comes with meaningful error messages and debugging capabilities. We speak both JSON and XML, and all the communications go via HTTPS so your data are totally safe with us.
APIs are cool, but sometimes all you need is to send invoices from a standard platform, such as Drupal E-commerce, Ubercart, Ruby on Rails, ZendCart, Heroku or Shopify.
We think it would be great if you could just use a plug-in or an extension and start invoicing right away. That's why we are already working on a set of integration tools for the most populars platforms. At the moment we are fully integrated with heroku. Check how you can install the addon and how to create invoices from your heroku application.