It's only a small job, charge what was agreed and hope that by demonstrating value and service they come back at some point for more larger ones. Do this enoug

It's only a small job, charge what was agreed and hope that by demonstrating value and service they come back at some point for more larger ones. Do this enough times and after a while you will have a steady stream of work from customers that you can work with on the basis of trust. PS : You say "your code saved them 4 times...", but sounds like it was their idea, so they could have got anyone to do it. Remember that, you should be grateful for the work.