The problem I see with that type of service is two-fold.
First, as you've already laid out, who's responsible for a death? Without any question it would be the customer. You're asking to QT a fish to ascertain that it is healthy and won't be a carrier of any parasites or diseases into your tank. Why? Because these things are an unknown to everyone. That's why you QT.
Second, and quite frankly more important. If you are putting a fish in QT, that fish, and any other fish added at the same time can be considered isolated. If another customer comes in a few days later and wants to QT their new purchase, if the store puts that fish into QT with your fish, the QT clock is reset to day zero.
Offering this kind of service with true individually isolated QT for every fish could potentially require a massive system. Huge expense. Your initial post had an 'lol' after the fee comment implying that you don't think that would be right. Just think of the potential cost for this type of service....floor space, tanks (multiple), heaters, lights, food, salt, RODI, labor to maintain, etc.