Buy dress online, alter in store?

So, I've tried on my favorite dress twice in a bridal shop, but the price was way out of budget. I'm going to buy it online, hopefully ebay (fingers doubly crossed!). I want to alter it at the original store, especially since they've made the changes I want and come highly recommended. I just wonder if they would alter a dress they sell but didn't buy there or would the service be bad... I feel so conflicted over it, but don't know another good alterations person. Unless any of you have East Bay recommendations...

I highly recommend Betty and Mary at Seams to Me in Newark. They are under Vendor Reviews:

http://www.projectwedding.com/vendor/show/10014115

HTH!