135612 Post Affiliate Pro Script Mod

I want to use ‘Affiliate Post Pro' ([login to view URL]) to handle affiliate sales on my website. I sell an ebook, and want to offer instant delivery. My site is a single page sales letter, and I want to use a ‘Buy It Now' or similar so it's very simple for my customers to complete their order. The integration instructions for Post Affiliate Pro require extra fields to be added to the Buy It Now button. This prevents the use of the standard button encryption feature. I've looked into creating my own buttons, getting my own keys, paypal's public key, etc, etc. However, there are elements to the integration that don't fit the standard method of encrypting your own button. Post Affiliate Pro requires the following line to be added to buttons: “input type="hidden" name="custom" value="" id="pap_dx8vc2s5” All that I've rerad indicates that only the “name” and “value” elements can be encrypted into the button. As it stands, the integration instructions provided by Post Affiliate Pro require the use of a non-encrypted button. So there is the obvious risk of people jumping to the ‘return url' and/or changing the price to $0.01. I'm looking for a SECURE way to integrate the script to my site. Post Affiliate Pro can integrate with PayPal's IPN, or it can use an ‘invisible img javascript' on the Thank-You page to register the sale. I ‘think' I need a php script to be written that will interact with the PayPal IPN. The script should check the correct ‘Price' has been paid. If this is correct, it should display the product for download and register the sale via the ‘invisible img javascript'. I would prefer the sale to also be registered via the IPN, but I'm not sure if the IPN can do both. Perhaps the Post Affiliate Pro Script that deals with the IPN notification from PayPal could be added to. It could register the sale as normal, but if it was set as the ‘return url' in the button, it could also present the product for download. This would avoid the problem of a sale being made, but the customer closing their browser without clicking ‘return to merchant' thus them not getting the product immediatley and the affiliate not getting credit for the sale. Alternative solutions required. Budget is small, but I hope this is simple! Many thanks. Mike