Puerto Rico is more expensive to ship to, it's not apart of the 50 states and it zoning is more since it's not apart of the 48 connected states, it's just a "colony" sort of....
I believe Puerto Rico is a commonwealth of the US, like we Canadians are part of the British Commonwealth. We have the queen on our money and postage, but we pay international postage whenever we ship anything to Britain because it's more expensive than to ship things domestically. ;-)