You can use URL cloaking.
I have done it by simply copying the content from another source, giving it a unique URL and allowing access that way.
You will have to keep track of the valid identifiers, and remove them after access, but other than that, the management is minimal.
So - when someone pays, you issue an id and save it in a file or database, or as a URL in a file - save it somehow. When the person accesses the content, you delete the id. If there is no id, deny access.
Good luck.