Sounds like you need to write yet another script 
I would just put some template-style variables (e.g., for the domain) into your scripts, then find an encoding application that runs on your own server (I noticed that LockIt! only runs on Windows).
You can use a full featured shopping cart, or just use one of PayPal's systems for the purchase. Once the purchase is verified, your script would just read in your script template, process it (e.g., replace ${DOMAIN} with the domain specified) save it in a separate folder with a unique name, then do a system() call to run the encoder, storing it in another (web-accessible) folder.
After that, shoot the user an eMail and/or present a direct link for download. The reason I suggest eMail, is that I have no clue how long the encoding process will take. You may just want to tell the customer to wait for eMail, or write a control panel script where the user can access previously purchased scripts.
Oh, and don't forget to charge optional additional fees for extended technical support, update subscriptions, and copyright removal  |
