Hello, and welcome. I am a licensed attorney and happy to assist.
I'm sorry to hear you are dealing with this frustrating situation. From the information you posted, you are entitled to return the mattress at this junction, and your reason is irrelevant. You don't even have to give one. That said, you may be subject to a 20% penalty which could legally be assessed because it is presumed that the company has processed the order, possibly paid for shipping, and incurred expenses in both time and money on it's end. If you canceled the order prior to any action being taken on the company's part, you may be able to negotiate a full refund with the manager, but either way you are not obligated to accept delivery and, if you refuse the mattress, at worst you will be subject to a 20% penalty. The rest of the money should be refunded. If not, you can take the matter to small claims court (assuming you do not accept delivery of the mattress) and get your money back that way, in addition to court costs, etc.
I hope this helps. If you need additional information or clarification, just let me know and I'll continue to provide assistance. If I have addressed your issue and/or pointed you in a positive direction, please let me know that, as well. Also, please remember to leave a positive rating when prompted, as that is the only way attorneys on this site are credited for the help we provide.