I've been very concerned about this too as we will be in St Thomas on a Sunday. From what I have learned thru reading another site is that yes, most stores will only be open half day. They will open anywhere from 9-10am and close anywhere from 1-3pm, so plan your day accordingly. How late they stay open depends on how many ships are in port. It looks like when we are there, March 12, there will only be one other ship besides the Golden. I doubt they will be staying open very late. Bummer.
My wife and I will also be there. We were concerned also being sunday what the hours would be. I emailed a store that my wife always go to when we are in port. They emailed us back stating they will be open from 9am until 1:30pm. The store is located in the downtown area. Hoope this helps.