I am not sure what you mean here. if you only want one item, at a fixed price, just create the correct PayPal button and add it in your sales page. The comment David had done, is that since you are using NON encrypted button code, this trick should NOT be used for downloadable products.
There are a few ways to have protection for your download link. The best, in my opinion, is to have a script that will check the payment, with PayPal, and if the payment is ok, it will display the download link ( attention: NOT the file name, just the link). Otherwise it will display an appropriate error messaGE.
I do have a script that allowes excactly this task: it is called IDC (Instant Download Cart) but it is a pro script and as such it is not free. If you are interested, you can contact me through
my form.