Update
Assuming I understand you correctly it is not that difficult to do providing you have been logical in arranging your store;-)
The format to select a catagory/product is similar to
Products_Detail.php?ProductID=7
So providing you can workout the product Id realtionship to your catagory/product you just need to make the code above your link (substituting your details of course)
As a total off topic are you using Paypal express? and have you got it working?
HTH
Ian