Software/Service Selling Q's

First.
I developed a "report center" app for a client. I want to to tweak it to be more customizable and then graft it to fit new clients data without rebuilding the wheel. So, do I sell them the "report center" as software and then charge for the time of grafting it to fit their data or... do I just rate the "report center" as X number of hours of work, then charge for the grafting portion seperately?

Second.
What if anything tax related do I need to provide to my client?
Nothing has been asked for yet, but I want to be prepared.

Third.
Any help appreciated.

(Moving to Gen Dev!)

First. What did they ask you for? Would they have wanted the tweak as part of the agreement?

Second.

Third.

The product I already developed won't be tweaked.
I just want to re-use it as a 95% done template for other potential clients. Simply changing certain parts of it to fit each individual client without rebuilding from scratch. So do I have to (or should I) sell it as a piece of software or rate it as X billable hours? Did that make any sense?
Thanks!

I personally think that you should rate it as x billable hours because then the client would know what to expect and what the cost is likely to be.