If you are coding a script for another web design company to use, how would you go about charging them for it?

Would you simply charge for how much work you think it will involve? Or would you charge an additional fee for how many websites they plan to use it on?

Licensing can a good way to build a residual income because you can build in a maintenance charge.

BUT you do need to build in a method for the application to "phone home" and verify the licence key and the location/site it is running in and licensed for.
It's not difficult with a bit of cryptography and hash generation.

Unless the script is relatively simple or can be assembled from your code library "guessing" at how long it will take to code is usually the best way to be selling yourself short

Thanks for the advice.

It is quite a simple script yes, which is making me wonder whether I should charge them each time they plan to use it.

If it were a really hefty piece of code then I would have to be more strict about it.