website integration for two websites with e commerce

I need a WEB DEVELOPER (NOT WEB DESIGNER ONLY) to integrate two websites I have. The first web site promotes a digital product and after clicking on a pay now button it goes through the payment gateway in order to direct customers (after submiting their payment information), towards a second web site where they can download the material I am promoting on the first web site. I need website integration capabilities that I don't have. Both web sites are fully designed and there is nothing to be changed in them. I only need the liason to be made so customers can get what they are paying for. I need this to be done ASAP. Thanks.