Password protection...won't work since if they are sharing the file they'll share the password, right...
So, you have to work out a scheme to use the Get(persistentID) tied in with a remote authorizaton file.
I woud think it woud work like this:
Customer buys file and gets a one shot authorization code from the site. The two files communicate and a table now has an authorization record created that says I am authorized for get(persistentID).
The next step is to ensure this record is created only once.
Now even if someone has a password your opening script will compare their get(persistentID) to the one stored in the authorization record and Telll them to Pass Go or Go To Jail...I mean Continue or Exit Filemaker or Close This File.
Of course there are variations on this idea but that's the concept.
No website? Then have them call and give them a dissolvable coupon code or something like that that is encrypted to the date, PersistentID and your present body temperature.