Did you use a credit card to pay for the fabric? If so, you can call the card company and stop payment until the issue is resolved. Same with a check. I wouldn't mind paying the $5.00 service fee to get my $45.00 back.
After you stop paying them, take the ride to the shop (make sure the manager will be there). Take the whole mess into her/him. It sounds as if someone working in the store is shorting customers and then stealing the difference without the boss knowing it. Stay calm, I think you can get them in the wallet. Worth a try anyway.