|Keep It Simple, Stupid
A URL does not have to map to a particular file name, so guessing what PayPal might have named its script (or handler, or whatever does the work) is pointless.
Note, however, that you usually don't have to name your script with an extension if it is in a CGI directory since the web server assumes that anything there is a script. The extension is useful when you allow scripts in any part of the browseable file system.
brian d foy <email@example.com>