Hi Shelly,
The Sleep Science mattress has 9" of latex which they say is Talalay. If they specified the ILD as 19 and 32 (which isn’t on the Costco listing) then it would likely be blended Talalay (you can see the different firmness ratings of Note: Footprint to Retired Webpage: latexfoam.com/latex-material/components . Their 100% natural doesn’t have specific firmness ratings (they use N1 - N5 to rate the firmness). did you get the ILD’s from Costco/Sleep Science?
2 nights experience isn’t really long enough to evaluate a mattress and I would give it at least a couple of weeks and preferably a month if possible so that the mattress has a chance to break in and you have a chance to adjust to a new sleeping surface. Of course, if the mattress is just a very poor “fit” for your specific needs and preferences in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) then sleeping on it for this long may not be possible. There is more about some of your options if the mattress is too firm in post #2 here but if the mattress is too soft then with a mattress where the layers can’t be adjusted or fine-tuned then your only option would be either turning the mattress upside down and trying to sleep on the firmer side (which would likely be too firm for most people) or returning it. A topper would only be effective if your mattress is too firm and isn’t an effective solution for a mattress where the comfort layers are already too thick or too soft (which generally indicates that the softer layers need to be replaced rather than using softer material on top).
I don’t have enough information about your body type, sleeping positions or more specific descriptions of your symptoms or what parts of your body you are experiencing them to make any meaningful comments but the first step is always to determine whether your “symptoms” are from pressure issues, alignment issues, or “feel” issues … all of which could have different solutions.
Some of the forum posts that talk about fine-tuning a mattress and may help you recognize or “diagnose” the underlying cause of different types of “symptoms”, pain, or discomfort that may be connected to a mattress include …
https://forum.mattressunderground.com/t/futons-looking-at-otis post #2 (this is the primary reference post for different “symptoms” that people may experience when they sleep on a mattress)
https://forum.mattressunderground.com/t/sealy-embody-introspection Post #45 (this is particularly worth reading as well)
https://forum.mattressunderground.com/t/purchased-a-plb-but-it-s-just-not-working Post #2
https://forum.mattressunderground.com/t/how-to-compensate-for-my-curves post #6
https://forum.mattressunderground.com/t/wondering-why-my-lower-back-is-achcy-in-the-morning-only post #6
https://forum.mattressunderground.com/t/fine-tuning-our-sleepez-10000 Post #4
https://forum.mattressunderground.com/t/sleep-ez-mattresses Post #7
If I had to guess and based on your comment that the problem seems to be limited to your back sleeping then I would guess that the comfort layer is a little too thick or soft for back sleeping and if this turns out to be the case it means that a topper wouldn’t likely be effective.
Phoenix