I think you are perhaps expecting a lot for what you paid.
I have a friend who underseals some quite exotic cars...is his only job. He looked under mine and thought most rust was very superficial. He could have done the job but would have been a big job, costing well north of £1000 to get into every nook and cranny. Would have involved removal of all superficial rust and then treatment with Dinitrol.
Did your quote involve rust removal or just spray treatment?