[FpUser]

>"Do you estimate there will be X hours of bug fixing and add that to the price of billable hours?"

I'll give you an example of particular product I made for a client:

The agreement I had stated that I would support (fix bugs, no feature change) for 3 month. After that it was $X per hour with $XX minimum charge. Alternatively client could always sign for a year of support at $xxx per year.

Currently I have a contract where they simply pay me flat fee per year with the clause that each side can terminate it with 1 month advance notice.

But in my long career I've had all kinds of contracts. Including getting licensing fee while I maintained and own product (I paid for development myself)