A framework agnostic, multi-gateway paymentprocessing library
Omnipay is a paymentprocessing library for PHP. It has been designed based on ideas from Active Merchant, plus experience implementing dozens of gateways for CI Merchant. It has a clear and consistent API, is fully unit tested, and even comes with an example application to get you started. You can learn one API and use it in multiple projects using different payment gateways.
...It provides a full-featured backend management interface along with customer-facing mobile and web portals. Litemall supports common e-commerce features such as product catalog, order processing, payment integration, user management, and statistical analysis, making it easy to customize and deploy a standalone online store.
Znode E-commerce Integrator is a C# Library for PaymentProcessing, Shipping & Accounting Integration. Currently supports Authorize.net, Paymentech Orbital and Paypal payment gateways. You can also retrieve shipping rates directly from UPS.