Sending mail (paper documents) recommendation

Hi everyone - I'm new to the islands staying in a long-term rental until construction is complete on a home. I see there is FedEx, UPS, DHL and the gov. post offices - does anyone have a recommendation on which is easiest to get a piece of mail (a few boat documents we forgot in the US)?

Also, how are things addressed to get here since it isn't like in the US where it is send to a home/business? I assume at the Post Office you get a PO Box, but the other shippers? Having trouble finding answers on the internets. Thanks in advance. Jennifer