For merchants doing custom development, Authorize.Net offers multiple methods of direct Integration to the Gateway. These are for merchants NOT using a pre-integrated Shopping Cart or POS.

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Here is a link to the Developer Playbook.

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Here is a link to the Developer API Reference Guide.

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Authorize.Net offers toll-free technical support at 877.447.3938 and supports more Shopping Carts and Point of Sale solutions than any other payment gateway.