Different shipping carriers require packages going to US territories (like Puerto Rico) to follow certain address formats and include varying order information in order to get shipping rate quotes and/or create shipping labels. In many cases, you'll need to fill out customs declarations, even though the carrier considers the shipment Domestic.
Below, you'll find the requirements for the USPS, FedEx & UPS as well as a list of the two-character abbreviations for the US territories.
USPS
The USPS requires shippers to use the two-character abbreviation for the state/region with the United States as the country.
With USPS, you will need to ship with a Domestic Service for US Territories.

Customs are also required when shipping to Micronesia with USPS.
⚠️ Note: For USPS shipments going to Puerto Rico Highway Contract Routes, the correct address line information follows this format "HC 1 BOX 18" --please eliminate additional designations, such as names of sectors.
FedEx & UPS
FedEx and UPS generally require the two-character abbreviation for the state/region and the full territory name for the country.
With FedEx & UPS, you will need to ship with an International Service with Customs information for US Territories.

⚠️ Note: You'll be shown international shipping services for UPS and FedEx shipments going to Puerto Rico; however, you will still be charged a domestic rate. This means that you will need to declare customs values for your order items as well.
US Territories and Their Abbreviations
Here’s a list of all current US territories and their corresponding abbreviations:
American Samoa (AS)
Federated States of Micronesia (FM)
Guam (GU)
Marshall Islands (MH)
Northern Mariana Islands (MP)
Puerto Rico (PR)
U.S. Virgin Islands (VI)
U.S. Minor Outlying Islands (UM): Baker Island, Howland Island, Jarvis Island, Johnston Atoll, Kingman Reef, Midway Islands, Palmyra Atoll, Navassa Island, Wake Island