I agree with Kevin. Make them show you how to fit the guitar in properly, or ask for your money back.

Did you happen to charge the purchase? In many states, consumer protection laws state that purchases made by CC are refundable should the goods or services rendered are not what is advertised or not in the condition as promised, you are due a refund. I'd suggest calling you CC carrier and ask if they accord that type of protection, and put them on notice you may be filing a dispute; they may immediately put it in dispute, which would give you leverage with the store- if they don't take the case back or offer you satisfaction, you can tell them, "well, good luck on getting paid." It's a hassle, but I hate it when people try to burn me.

Good luck.