How to sell some PHP code?

I've been developing a modification for my shop site.

Mod's are quite common for cube cart shopping carts.
However due to the fact that code relies on an existing script the user has to insert bits of code etc, again this is common.

I'd like to provide a working version for people to try.

But I'm going to have difficulty adding any sort of protection to the code as its bits here and there.

I can't provide full files as I'm re-using (copying) a script provided in cube cart and its not open source.

Any suggestions?

Also, if I overcome this obsticle I've got to figure out how to provide licenses etc, if needed. And how to accept payment, alothough a paypal button might do.

I've developed add-ons for software etc in the past and I've never made much money from it so I'm not really able to put much effort into the process, so things will have to be simple.

by JM