If the STATED size was 18x21, it was your responsibility to make the decision to buy it or not.
On the other hand, if the pieces were cut crooked, I would have returned them immediately and asked for (and gotten) a refund. That's just sloppy workmanship by their cutters and there's no way I would accept it. We quilters are a loving and forgiving group, perhaps its time we show a bit of "backbone". We were successful with Walmart, pehaps its time other merchants heard our voices when we are displeased.
QUILTERS OF THE WORLD - UNITE!!! lol