Communications
The most reasonably priced way to send things to the island, is to use the US Postal Service. You can mail letters back to the mainland with a regular US stamp. It is best to use Priority Service to get shipments to and from the mainland in about 3 days.
Most major cell phone carriers operate on the island. If is best to check at home with your carrier, to verify the cost to call or text.
There are a variety of channels on the cable system at the complex.
There is internet service at each condo, and some hot spots around the grounds.
Electricity
St. Croix uses the same electrical plugs and voltage as the mainland US. If you are bringing sensitive electronics, there are sometimes power outages, and a surge protector can be a good idea.