Hi bluemoose,
You can see my comments about Bedinabox in post #2 here. They use 3 lb memory foam which is a lower quality and less durable material than I would suggest in this budget range.
If the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know for certain whether a specific mattress/topper combination is a good “match” for you in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) is based on your own careful testing or personal experience on the combination. If you can’t test the combination in person then there will always be always some risk and uncertainty involved in adding a topper because the specifics of 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.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable 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. A good exchange/return policy can also reduce the risk of an online topper purchase so I would make sure you are comfortable with the options you have available after a purchase just in case the topper you choose doesn’t turn out as well as you hoped for.
Having said all that … because of the uncertainty involved in choosing a topper that will work well for both of you and because of the quality of the materials in your mattress … I would tend to avoid trying to “fix” your mattress because Bed in a Box has a good return policy which lets you start over again to find another mattress that will be more suitable for both of your needs and preferences.
There is also more information in the first part of post #2 here about some of the different ways that can be used to accommodate a couple that have very different body types or sleeping styles or that for other reasons have different needs and preferences in a mattress.
If you do decide to return your mattress and try again then the best place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that you will need to make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort” and PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists (based on all the parts of your personal value equation that are most important to you).
If you are considering online choices then the mattress shopping tutorial includes several links to lists of many of the better online options I’m aware of (in the optional online step) that include many different types and categories of mattresses with different designs, options, firmness levels, prices, and return/exchange policies that may also be worth considering.
If you let me know your city or zip code I’d also be happy to let you know about any of the better options or possibilities I’m aware of in your area.
Phoenix