I don't feel you should be held responsible. Once the owners picked up the dogs, they became their responsibility. I can't fathom forgetting coats for their dogs if they are used to using them. The only reason you might feel you should pay is that they might really badmouth you and try to ruin your reputation if they are that kind of people.
I don't know if I would pay much heed to gossip if I was looking for a boarding kennel though, most people visit and check a place out and make up their own minds.