Add Google Checkout Button to RSS Feed

I have already an application which generates a RSS feed for a database of products. I need to add to this RSS feed the html code for each item so that it complies with Google Checkout Button. Each item is a product in my database and each needs to get a customized button form code - the steps involved will be Create a checkout the item as an XML structure. Create a signature for the shopping cart using the HMAC_SHA1 function. Base64-encode the cart. Base64-encode the signature. Put the cart and signature into a Google Checkout button form. This is exactly the process Google uses to create checkout buttons, but Google only allows this done by a human operator, I need this automated. Sample code at [login to view URL]