From their site: PaySimple not only enables you to easily process electronic check and credit card payments from your computer, it also proves a gateway for your customers to initiate payments.
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There are no mashups, developers or code associated with this API yet.
there is a C# client here : https://sandbox-api.paysimple.com/3.00/help/default.html
I developed a Java client for the PaySimple API. Contact me if interested.