they're just bratty at that store it sounds like. I go into the store by my house a lot - suburbs of Dallas/Fort Worth (at an outdoor shopping center with high end restaurants and stores, Pottery Barn, Crate and Barrell, Cheesecake Facotry, Anthropologie, etc), quite often I've been in a tee shirt and shorts, even sometimes carrying a Michael Kors bag that I carry a lot, and the girls there are always super nice to everyone.
Poo on those girls at Providence Place! Try another Coach store (maybe up in Mass? I haven't lived there in a while, but I'm sure there has got to be a coach store in SE Mass without having to go up to Natick or Boston)
I'm so sorry they treated you like that. It shouldn't matter what you were wearing or carrying. They should treat everyone with respect