The big challenge is that you didn't buy the tires from them. Many places won't mount tires they don't sell, and the few that do aren't as motivated to be all that helpful.
I very much don't have a fear of heights. As a kid I used to do a fair bit of rock climbing, and worked as a lighting and sound tech for a while...I've spent a lot of time way up in the air. The only real treacherous part was the getting back onto the ladder after doing the work.