Kerry Francis offers child counselling services via www.letstalk.co.nz/. She asked me about providing a zip file download for sale via Paypal. The zip file contains PDF activity files and audio files, though for our download purposes all zip files are the same.
The site already used a Paypal basket for off-line sales, so it was an elementary clone to get the new item displayed on the page and the Paypal button working. But the main obstacle was offering a zip file which could only be unlocked by the purchaser, and in addition, would only be available for a certain time period. This narrow window of availability helps protect against web-bots or other villians scavenging the zip file and circulating links for free.
I could have developed something from scratch but clearly this is a generic problem, solved many times by smarter people then me. A little research uncovered LinkLokIPN which perfectly suited our requirements. So my task was the setup and testing of the script. It needed an elementary bit of computer knowledge but worked perfectly, and I recommend it.