We are going to have some appliances and other bulky items shipped to St Thomas from the USA. I know of Tropical Shipping and Crowley Marine. Is it easy to deal with them direct or is it better to use one of the agents in St Thomas to deal with the logistics...?

Call St Thomas Cargo. They do freight forwarding from Ft Lauderdale. They can handle customs and local delivery as well.

We just shipped our new paddle boards with Troplcal from Miami. Pretty reasonable for shipping and duty. The process was onerous on both ends.

We've used Tropical twice for shipments from Canada, via Miami and done the clearance ourselves. Quite easy, if a bit time consuming. You go to Tropical first to pick up your waybill, then the VI tax office. From there, Homeland Security (Customs) and finally back to Tropical with your stamped documents to collect your freight.
The better part of half a day, mind you we weren't on a schedule of any kind.