What exactly are you worried about? Not having the shop that you bought the dress from do your alterations? Usually when places say that alterations are included in the price of the gown, it's probably because they've just raised the prices on all their gowns. Finding the perfect dress is most important, AND you saved $500! Now you just have to search for a good seamstress elsewhere.
When I was dress shopping, I wouldn't tolerate rude, pushy sales people. If they seemed snotty, I just left without trying anything on because I am very intimidated by people like that. The store I bought from was sooo nice and I felt really at ease there, plus I liked their selection. They do alterations there, but if they didn't, I would still buy there and worry about finding a good seamstress later.