What should I know about “selling” my first software?

I produced a piece of software aimed towards a non-profit for my capstone in my CS degree. It turned out nicely, and the company wants to pay me for it. It’s a non-profit with a great goal, so I don’t have any interest in attempting to get a lot of money, but I do want to make sure I do everything “right”. I also want to potentially give/sell this software to other non-profits that would have a use for it. What are some tips or things I should know about this to avoid industry mistakes or make the process easier?