I told them that I am not going to charge for parts for ovbious reasons
are you sure that you don't mean $100 call out charge, ten parts on top of that?
if you say $100 to fix any problem what are you going to do if a CPU has died, or if you need to buy more RAM? or a new PSU. I'd have taken the $75 and said that parts (if needed) are extra.
and you'll find that this is the best way to start a computer business, I've had a few people ask me if I'd start up a computer business specifically because I don't rip people off.
(the last time I did this was I went to a friends dads business and showed them how they should install the RAM in their PC, after seeing how easy it was to just push the stick into the slot they decided that they should probably stop using their current IT guy as he'd charged then a hundred quid, (£100, not $) for the same job. -I had said that they should just pay for the RAM, and I'd do the work as a favour.