Can someone tell me?
Costco. But you may spend more than $125. My handy wife and her friends also bought a bed frame
I think we spent $250 for the mattress but it’s an incredible bed 
. Since my wife took six hours to build it, it was an inexpensive bed frame
I have noted that the Bed in a Box folks all seem to have a good return policy.
We have always had the folks delivering the mattress take away the old. That does not seem to work if you are relying on the return policy.
Just wondering how this is handled. Ant experience??
I can’t give you an answer but can help you think through the process. I worked in the mattress industry for nearly 20 years. Bed in a Box is a foam mattress so you’ll want to look for the Indentation Force Deflection (IFD). The higher the score, the firmer the mattress. Firm isn’t always better, you need to make that call. You can always go into a local store and test some mattresses to see what firmness you like.
If the mattress is memory foam, it’ll hold heat, if you sleep hot, look for open cell. I’ve owned one of these and found it to be incredibly comfortable for a year or two and then quickly deteriorate. The one I owned was a prototype that is no longer in production.