Hi tm820,
Wow … that’s amazing … and very unusual!
I think it’s probably past its due date 
The best use for a topper is for a mattress that is too firm and needs some additional softness and pressure relief but it’s only likely to be an effective solution if your mattress is still in good condition and there are no soft spots or sagging in the mattress. I don’t know which materials are inside your mattress but a topper isn’t the most effective solution for a mattress that needs to be replaced.
I don’t know which Costco topper you are referring to (there are many gel memory foam toppers on the market) but the biggest factor in the quality/durability of a foam material would be its density (see the quality/durability guidelines here).
Just in case you haven’t read it yet … the mattress shopping tutorial here has all the basic information, steps, and guidelines you will need to make the best possible choices … and perhaps more importantly how and why to avoid the worst ones. If you follow the steps one by one you will have the best chance of success. I would keep in mind that who you deal with in terms of their transparency, knowledge, and experience can be one of the most important parts of a successful mattress purchase.
The better options and possibilities I’m aware of in your area are in post #2 here and the two manufacturers in the Washington State list that it links to.
Phoenix