There is programming involved in the solution that you're looking for.
I spent 6 hours with my developer figuring out how it all worked and still didn't end with a viable solution like your looking for.
Essentially, someone could change the price on their purchase from you and still land on the download page and pick up your product.
The optimal way for it to work is.. Your script sends variables to paypal through the payment process, once payment is received, paypal sends that payment has been received back to your script, your script then sends the information back to paypal requesting confirmation of the purchase (including the correct amount), and once confirmed or denied, paypal sends that information back to your script which in turn takes the appropriate steps to getting the product to your customer..
I hope that makes sense..
1. I want the product --> pay paypal
2. Payment received --> back to your confirmation script
3. Did they really pay the correct amount and here's their payment ID for you to check --> to paypal
4. They did/did not pay --> to your script
5. Ok.. they paid, here's the download page... or no, they didn't pay.. "Nice Try. I'm keeping your money now try again!".
How can you easily automate this process?
Setup a clickbank account, build the pages that they tell you to. Let them handle the backend and pay them a little on each purchase and pay affiliates a commission for selling your product for you.
In Honest Riches, Holly mentions another company like clickbank. Both accept paypal payments I believe.
I believe this to be accurate, but if I'm off, someone please correct me