Hi mikes425,
Did you have the chance to try the mattress before you purchased it?
If you purchase a mattress online then the exchange/return policy and the costs involved can be one of the most important parts of the āvalueā of an online purchase just in case your choice doesnāt turn out as well as you hoped for (although as you know if the mattress is too firm then you also have the option of adding a topper).
Your mattress is a one sided pillowtop and would do best on a firm, flat, non flexing support system such a foundation or platform bed that has minimal to no flex rather than a box spring that has springs inside it and is designed to provide flex under the mattress so hopefully the support system that you kept is a suitable foundation rather than a box spring.
I would give your mattress a few weeks so that it has a chance to break in and you have the chance to adjust to a new mattress that is different from what you were used to before you consider making any changes or additions to the mattress (see post #3 here and post #2 here). If after a few weeks you still find that you need some additional softness and pressure relief then post #2 here and the topper guidelines it links to can help you use your sleeping experience as a reference point and guideline for choosing the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success and also includes a link to a list of some of the better online sources for toppers Iām aware of as well.
You would have several options that would probably work but it will depend on the specifics of the bedframe and the foundation that you currently have.
If you have a standard metal bedframe (which is usually about 7" - 7.5" high) then you could replace it with a low profile foundation that is several inches lower.
If you have a box spring then I would consider replacing it anyway but if you have a foundation that is a standard height (generally about 8" - 9") then you could replace it with a low profile foundation (generally about 4" - 5"). I know that Charles P Rogers sells both a standard 7.5" and a low profile 5" foundation as well that would work well with your mattress.
There is also more information about the types of support systems that are generally most suitable for different types of mattresses in post #1 here and it includes some links to some examples of each of them as well.
Phoenix