409404 paypal pro integration

We currently have a copy of xzero classifieds that needs to have paypal pro integrated into it. We would prefer that when they click pay that it redirects to one main page to bring the whole site together as we are using multiple instillation of the script on one site. Whom ever we choose for the project should be able to integrate it into the script and give us detailed instructions on how they did it so we can ad it to our modified copy of the script. If you have any questions of concerns feel free to contact us.