I assume that this charge included the cost to fix the leak problem AND gives you some sort of warranty for the next couple of years against any leaks?!?
Your home air conditioner is not like the one in your car, and ideally should NEVER need a top up of refrigerant unless there is some unusual situation, like you damaged a refrigerant line while moving something, or the original installation was shoddy.
Consumers are used to a garage having to top up your air conditioning, so some home HVAC contractors take advantage of this and use the same excuse. Unless you are rattling your home AC the way the one in your car gets rattled as you drive around, it just isn't going to be low on refrigerant.