The end of wallets?...in the future?...Is this like some sort of fanboy thing to explains why there's no pockets in Star Trek?

Reconsider getting into this. Here's the issue: as soon as you take money, you are the guy who was paid to fix the computer. At that point, when you walk out the door, the person is going to immediately install a bunch of Spyware and basically undo all your hard work.

...And call you back complaining their PC: (pick one or more)
A. Still isn't working. (inference: You didn't do anything)
B. Isn't working again. (inference: You didn't do enough)
C. Is worse then it was before. (inference: You did it all!)
D. Is missing the spyware loaded software that you told them not to run because it was causing the problem.
E. All of the above.
F. Any, or all, of the above plus they need you to fix it RIGHT NOW because they've got a deadline... and "20 minitues should be enough time to fix it right"?

The problem I've run into is that once you touch the PC, especially if you take money... or ever a coffee, is you become their de facto tech support.... And there's nothing like a 3am call because their kid has a term paper due in the morning.