Why would anyone be offended to leave a deposit? , do you not ask for a deposit for a commissioned quilt.. ?!?!?!?! OF COURSE YOU DO .. why? so if the other person changes their mind you still cover your cost..
No different in this case, sure they have YOUR machine, but anything could happen, and the owner of the machine does not return, lost your job- no money to pay for the servicing, God calls.. Cash pays the bills, not your machine sitting in the back collecting dust..
the shop already forked out the pay for the servicing..