[cat9]

This is a good reference implementation.

https://news.ycombinator.com/item?id=7850335

In general, I would avoid selling software without a service agreement.

Bugs are a thing that happens. You will endeavor to make them happen less, because you're a competent professional. But because you're a competent professional, you know that some will happen anyway. So structure your business deals such that both you and your customer have a good answer to the problem. You shouldn't take a wash for being available, and they shouldn't be left hanging with nobody to get help from.

This gives you a tidy source of recurring revenue and an easy way to nudge up your average deal value. It gives your customers a reliable source for fixes and changes. It's good business all around.