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From their site: Since 1996, Authorize.Net has been a leading provider of Internet Protocol (IP) based payment gateway services, enabling merchants to authorize, settle and manage credit card and electronic check transactions from a Web site, retail store, mail order/telephone order (MOTO) call center or mobile device. All you need to authorize and process payments through Authorize.Net is a live Internet connection and the ability to emulate an HTML Form Post using a Secure Sockets Layer (SSL) connection.
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I have been using free Oscommerce shopping cart with Authorize.net the last 5 years, which very easy to setup. It is only $7 at www.unitedbankservice.com if anyone is looking for an inexpensive solution.
the authorize.net API is quite simple to use. We at menexis have been utilizing their platform for years now. They also have a great support system