Hi Xellon,
Ditto to what brass said about the platform bedframe (thanks brass :))
I would also want to make sure that it has good center support to the floor (at least one leg in the middle supporting the midbeam).
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here and I would also want to know the type and quality of all the materials in the mattress to make sure there are no weak links in the mattress. While they are not particularly knowledgeable about mattresses so they can’t really give you good guidance about whether the mattress would be a suitable choice for your parents … the only good thing about buying a mattress from Costco is that you can return it their sleeping experience indicates that it’s not a good match for them in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). There is more about buying a mattress from Costco in post #4 here.
Phoenix