True, it would be a nice gesture for the shop to help out. That being the simplest way to handle it; call, alert the shop of the problem, problem corrected, end of story.
However I wouldn't get comfortable thinking that's the way it will be. You were there, purchased said fabric, left the store. By that, you accepted the fabric "as is". So the shop, if they take the stance, has no verifiable proof that you ruined the fabric in some way and are trying to scam them for more. And more importantly, you do not have the proof to refute any such argument.
But as most have pointed out, the majority of shops will do what it takes to help out and find an agreeable solution. Hoping the best for you.