On the subject of adding credt card transaction capabilities to windows and web apps, below are a couple of articles that I helped author. The articles show how using /n software's third party toolkits to make the implementation very simple.
IBiz E-Payment Integrator can be used to add credit card and electronic check transaction processing to your application by communicating with major Internet payment gateways.
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IBiz VitalPS Integrator can be used to add credit card transaction processing to your application by communicating directly with the Vital Processing Services processor.
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