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