Hi Diddlydo,
There are many people in the forum who have purchased a mattress online and done very well including many who have bought from some of the members listed here. All of these are very knowledgeable, have good quality and value, and will work with you to help you make the best possible choice.
Costco sells many mattresses so it would be helpful if you could link the mattress you are considering or at least provide the make and model you are looking at. To me the “best” thing about Costco is their return policy which lowers the risk of buying a mattress from somewhere that knows nothing about them but I wouldn’t consider any mattress … at Costco or elsewhere … where I didn’t know the details of the layers and materials inside the mattress because it’s the only way to make meaningful quality and value comparisons with other mattresses. The staff at Costco know nothing about helping their customer “match” a mattress to their needs and preferences so you are really rolling the dice when you buy there which is why the refund policy is so important. No mattress is worth it at any price if it isn’t suitable for you in terms of PPP (Pressure relief, posture and alignment, and Personal preferences) or uses lower quality materials that won’t last you long enough to justify the price.
I haven’t heard you mention some of the better options that are in the area that are on the St Paul/ Minneapolis list. Have you visited them? I place a very high value on who you buy from not just because of the quality and value of their mattresses but because of the knowledge they have and the guidance they can provide you. There are some very good choices on the list that would be well worth a phone call and a visit even if they involved an hour or so travel time IMO. It would probably save you time in the long run and you would end up with much higher quality/value and not have to do it all over again nearly as quickly.
It just seems unusual to me that you are thinking that you are running out of options but you haven’t mentioned some of the better choices on the list that are available to you.
Phoenix