With an individual account you have almost no control over shipping.

If a buyer from Puerto Rico places an order -- for many many years Amazon treated that order as international if the buyer entered Puerto Rico as the country. If the buyer entered USA as the country the order would be treated properly as a domestic order. I think this may still function that way -- from a few threads in here is seems like Amazon has never corrected this.

If you are on the individual plan, allowing international is a bad idea (I think it's a bad idea anyway on Amazon) -- but especially for those with the individual account.

Orders to Puerto Rico should be treated as domestic, and any USPS flat rate packages will be the same as other, but other carriers are going to be a lot more expensive.