I'd say be honest with them and rather than just bill extra hours, bill a flat fee for use of this code base and then bill for each hour spent customizing. I th

I'd say be honest with them and rather than just bill extra hours, bill a flat fee for use of this code base and then bill for each hour spent customizing. I think any reasonable client would find value in having code already written that can be used for their project. Even if the project is open source, I'd still bill a flat fee for inclusion of it in a project given that you have very high expertise in it and thus can be more efficient and effective with customizations than an unfamiliar programmer could be.