An interface for an integrated POS solution that allows a merchant to process card payment transactions through a secondary transaction payment processing network. The interface of the present invention expands the functionality of POS system by expanding the scope of use of a secondary port in the POS system. A primary POS port connects to a primary transaction payment processing network and a secondary POS port connects via an interface to an Internet payment gateway and then to a secondary transaction payment processing network. The interface converts the secondary port message formats into a message format for processing through an Internet payment gateway and secondary transaction payment processing network. The interface also performs additional reasonability and security checks typically not performed by a secondary port on cardholder track data. The improved processing of track data results in fewer declines so that transactions are processed more efficiently and accurately.