Well you get it at a store you didn't purchase from it is very possible they'd just tell you it is FAKE regardlessly. I've seen and heard plenty of case like this.
Because you didn't purchase from them so therefore they are 'losing' money and would not help in that case.
I've had a friend who took a bag she bought from Neiman to authenticate. Walked in and said wasn't from there, the SA immediately said then this probably fake. Then she took out the receipt and said it's from them, and the story changed.
Find an official authentication service to check it, that's what I'd do.
