I recommend telling the shop owner exactly what you've said here. It may be the owner already knows the situation, but it might not be the case.So, I bought some old gear and had it serviced by a LDS. I told them that the BC was self inflating, they made a note of it, shipped it off and I got it back in a week, which is standard around here. I got the BC back without any old parts attached even though the receipt said that parts were changed out (second time this has happened to me with this shop). I took it home and a couple of days later I decided to hook it up to a tank and make sure it was fixed. As you've probably guessed, the problem wasn't fixed. So I took a couple minutes to calm down and called the shop to say "WTF??!!" in the nicest way that I could. They told me to bring it in so they could check it out, after quickly determining that it wasn't my fault, they sold me a new inflator minus the cost of parts, (looking back on it I am not sure what I ended up paying labor for...) anyways, the dive shop manager asked me if I wanted to send the old inflator up to the shop (at no cost of course) to see if they could fix it, one week later I received my original inflator, complete with new O-rings in "working" condition. I have yet to test it out.
Just wanted to put this out into the SCUBAsphere and see what bounced back, I am not really sure how pissed I should be, or if I can really trust anyone to service my gear anymore... I don't think that, had the roles been switched, I would have handled it they way the shop did. I am pretty sure they wont see another dollar from me, and I guess I will just have to take this as a lesson learned; even clean looking shops with nice employees cant be trusted by appearance alone, also cheaper does not equal better.
That said, I'd never pay anyone to service a BC in any way shape or form. They're too easy to inspect and service yourself. I think you let them off too easily, to be honest, but then you were there and I wasn't. If you have another shop to deal with, that seems like a good choice.