Hi ohaya,
You can read m thoughts about buying from Costco (or any of the big box stores) in post #4 here. You are really rolling the dice when you make a purchase from a store that knows little about mattresses or the quality or suitability of what you are buying for your needs and preferences. In most cases the quality/value of most of the Costco mattresses are also not the best available to most people. As you mention though at least you get a “free roll of the dice” when you purchase from them because of their great return policy (although they still need to assess the return to make sure it is reasonable and that there are no stains on the mattress).
To me the quality/durability of a mattress (based on the quality of the materials inside it), the suitability of a mattress, the guidance available from the manufacturer or retailer, and the quality of your sleep, along with your ability to make custom choices, to fine tune or make adjustments to a mattress instead of returning the whole thing and starting all over again, and higher quality and performance materials and better value are more important than a return policy but of course each person’s personal value equation may be very different.
If you do decide to go in this direction then the same information and guidelines that are in post #1 here that apply to any purchase would apply to a Costco purchase as well.
As you will read in the “read first” post … I also wouldn’t walk through the front door of most chain stores and I also wouldn’t consider a major brand mattress where there is no way for you to know the quality of a mattress you are considering (they don’t disclose meaningful information about the quality of the materials in the mattress).
The Stearns & Foster Cora has 4"-5" of lower quality polyfoam in the comfort layers in a one sided mattress depending on the version (see here or here) but they are both the type of materials and construction that I would specifically avoid.
As you can see in mattress firmness/comfort levels in post #2 here, I can only help with how to choose (not the specifics of which specific mattress to choose) and speak to the quality or value of a mattress. There are too many unknowns and variables involved for anyone else to know which mattress would be most suitable for you without either your own personal testing or the experience, knowledge, and guidance of the manufacturer or retailer you are purchasing from who can help you choose between specific mattresses they make or sell based on the “averages” of other people. If you do choose to purchase from Costco, I would only consider the mattresses where you know (or can find out) the quality of every layer in the mattress so you can make sure there are no “weak links” in the mattress (such as the use of thick layers of lower quality polyfoam in the Cora) meaningful quality and value comparisons to other mattresses.
Brooklyn Bedding is one of the manufacturing members of this site which means that I believe they are among the best quality/value in the country. They are also completely knowledgeable and transparent about the design and materials they use in their mattresses, they provide comfort and support options before and after a purchase with many of their mattresses, and you would have all the information you would need to make informed decisions and meaningful comparisons with other mattresses.
Phoenix