You know what? 95% of your trouble tickets will be generated by 5% of your clients. That's a simple fact. They will drive you out of your mind with the stupidest problems imaginable, eating up time that could otherwise be spent on actually generating money. ("Why isn't my PHP page rendering? Oh, it was my mistake? Why do I have to pay you to troubleshoot that?")
If you want to be able to pay your bills, either set this up as a write-off associated with something that actually generates income, or just stay away from it altogether.