I've got to side with the OP on this one. As the owner of a retail store, mistakes do happen. But when its your fault you must take nearly any reasonable measure necessary to fix the problem, even if it cost you money or time.
I'm always saying that i have to do other people's job for them. MS's job was to weight the disc, confirm the weight and relay that info to the customer. Somewhere they failed and customer had to do all this for himself. Then the customer offered to help them fix the problem by spending as much as it would have cost MS to drop of the disc at FEDEX.
Also, a top rule in customer service is never to get to the low that was obviously reached here. When you antagonize the customer by saying "what does it matter" you're just making things worse for everybody. You're the manager, you need to tell people your name when they ask. responding to the "man up" comment brought this costumer to be upset enough to spread bad rep.
I've had customers kick, scream, curse, and use just about every four letter word. But the moment you get in a chest match or start responding to insults with insults...you have failed as a manager.