20% Off Casper 12" Foam Mattresses (All Sizes) | Amazon | Some require you to clip a 10% coupon
Casper offers 10% discounts on its own site from time to time, but oddly enough, you can get much better deals on the original mattress in a box if you buy yours from Amazon right now.
Every size of the standard 12" foam mattress is about 20% off, with Prime shipping. Some, like the queen, are marked down by the proper amount already, but others will require you to clip a 10% coupon on the page to stack with the existing discount.