Hi DCGiggles,
I hope that your expectation is different from your actual experience but either way I would make sure that the mattress you received or that is on your receipt is the same as the mattress that you tested in the showroom (by checking the law tags). I also hope you have the chance to let us know what happens.
If your mattress is still in very good condition and there are no visible impressions or sagging or soft spots in the mattress and the only issue is that it is too firm then a topper can certainly be a good option to add some additional softness and pressure relief although there is always some risk and uncertainty involved in adding a topper if you haven’t tested the combination in person because the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
One of the benefits of adding a topper is that it is much less costly than a new mattress and if a mattress/topper combination turns out to be a good “match” for you in terms of PPP then it also has the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (the upper layers or a sleeping system tend to soften or break down before the deeper layers) or if your needs or preferences change over time and a topper can also help extend the useful life of the mattress underneath it as well.
If you do decide to try a topper then there is more information about choosing a topper in post #2 here and the topper guidelines it links to about firmness and thickness which along with a conversation with a reliable supplier that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better online sources for toppers I’m aware of and a link to the online suppliers that have good exchange/return policies as well.
Phoenix