First of all, I absolutely think you should have been able to return the mattress. A company with as many resources as Wal-Mart should be able to time their printed material to coincide with any major policy changes. I know that in my time working in retail we got new signage, register paper, and brochures well in advance of any policy change that affected customers and just waited on the date of the change to put it out.

Considering that this new policy is not on the receipt (which functions kind of like an agreement between seller and buyer), they should have taken back the mattress in my opinion. Similarly, if the receipt HAD said no returns on opened packaging, I'd have said you were out of luck.

If there were in reality no changes to the return policy, then I agree that you were treated unprofessionally by the employee(s) you spoke with.

So I'm with you to that point.

Here's the thing, though. When you write a letter like this to a business, you are asking them to take some time and review your issue individually. They may or may not be willing to take that time, and they are made less likely to do so by wording like you use in your last paragraph. It's a shame, because I was totally with you until then. Being insulting to someone you are trying to persuade is never a good idea (even if you really are right), even leaving aside questions of rudeness. I understand that you are angry, but emotional language and threats should always be left out of a business letter.

Good luck!