As someone who cuts fabric in a quilt shop, I have another perspective. The quilter always has a clearer idea in her head of what she is doing than I do as the person in front of her cutting fabric and not everyone can clearly express what they are doing. I just think it's bottom line the quilter's responsibility to figure out their yardage, at my shop we ALWAYS help people with that, but I do get frustrated with people who don't even attempt to learn how to figure out their yardage. Just another point of view.

I would take the fabric back with the quilt top and tell her what happened.

IMHO it wasn't the shop owners responsibility to figure out how much you needed. She tried to help you.

I wouldn't even mention it.

I think I'd agree that you should take it back to the quilt shop and explain what happened. They may be upset that it's washed already, but I'd sure try.

I would make the outer border slightly smaller!

Never let her calculate fabric for me again. That's what I would do. LOL. I would go back and tell her she was off and now you are stuck with a backing that you can't use because of it. If she does not make her mistake good then never shop there again. If she does make it right then pay for 1/2 of the new backing cost. To me that is the best thing to do. Everyone makes mistakes.

I would go back to the quilt shop and see if she would take it back, if not try to sell it on QB. Good luck.

I would tell her what happened and ask her for some coordinating fabric to piece the back. I often make a strip along one side for the label. I think it adds some class.

go back and blow up her shop? maybe not - just go talk to her...she won't want to lose a good customer and l'm betting she'll right the wrong!!

How about making corner blocks to make up for the fabric shortage (I've done this and it works well)? I wouldn't mention it the store - I see too many negative possibilities:
... you misunderstood what I said;
.... next time do it yourself;
.... I don't remember the incident, you sure it was me?;
and on and on and on.....
Chalk it up to lesson learned and next time come here for help. I'm sure "Craftybear" could have come up with the correct answer in a flash, probably with several websites to back her up LOL