HI cassandram,
Welcome to the Mattress Forum! 
The best advice I can offer you is something to which you already alluded – give the mattress more time. You’ve only had it for a few days and it takes time for your body to adjust to the mattress, and for the mattress to adjust to you to (see post #3 here).
One thing that you may wish to investigate right now is your pillow. Issues in the upper back with a new mattress and the shoulder area are commonly the result of using a pillow that may have worked for on old mattress, but with the way you sink into the new mattress may be different and require a pillow that is thicker or thinner than your old one to maintain a more neutral alignment.
I would combine the pillow analysis along with attempting to get used to your new mattress, and if you do decide to return it, then I would begin your mattress shopping process again, starting with the guidelines in the mattress shopping tutorial. If you have questions at that time, we can revisit your specific needs, but I wouldn’t want to focus on that right now, and instead give your current mattress a more complete testing.
Phoenix