Yahoo! Store Module or Modifying Script

I need to slightly alter what occurs when a customer uses PayPal Express Checkout on a Yahoo! Store. IMPORTANT: I need these alterations to be "installed" on multiple stores, so I'm looking for more than a one-time change ... I need an installation script or a module that makes it easy for other store owners to make the changes. Here they are: After the shopper authenticates at [login to view URL] and is redirected back to the Yahoo Store, I need the PayerID (obtained via the GetPaymentDetails() API) and SellerID (supplied by the person who installed the module or ran the installation program) sent to a script located on my server. The script on my server will provide a JSON response. The data in the JSON response will be used to create a discount ... this means two things: 1. The discount must be "created" in the shopper's cart so that the shopper has visual indication it was applied ... e.g. create an item with -$ (Description), and 2. The final $orderTotal sent to PayPal via DoExpressCheckoutPayment() must be reduced by the value of the discount ... so the amount the person is charged is actually discounted. Please view this presentation: [login to view URL] before you bid.