The CheddarGetter Python SDK by Sean O'Connor interacts with the API to access billing data, useful to integrate in optimized web services. Sharpy, the name of the SDK, provides BSD style license for the open source project, installation with Pip, and implementation with the majority of the API resources.
Online bookseller Erapublications has an API for its in2Era web service for selling ebooks through online distributors. According to its documentation, this RESTful API, which requires an authentication key, integrates its catalog onto your site. Distributors get a cut of the sales.