Right now I don't charge but I am considering it - but only to cover costs for materials i.e. swim noodles, locking clips, anchor bolts, extensions etc. Thankfully my instructor gave me a good stash of locking clips so I'm ok for them for a while yet!
Yesterday I had a dad come to me to have his seat installed, poor guy looked at me like I was insane when he asked how much it would be after I finished installing his seat (gave him a free swim noodle and locking clip) and I said it was nothing - free. He asked me two or three times and was shocked when I said no charge - just spread the word! LOL
Fortunately I don't really have to worry about liability issues here in NZ and even if I did charge, I think I would feel far too guilty to charge more then materials! I have had people tell me I should offer my services (paid of course) to others - maybe in the future!