We never said 'once it leaves, it is not our problem'. We said that once it leaves, we cannot control how USPS routes the item, as it's in their hands.
Please see the exact response we sent you:
Once an item leaves us, we have no control how USPS chooses to fulfill it through their network. Puerto Rico, like Massachusetts, is part of the United States.
It might seem a bit strange that they sent it there first, but it’s probably because they have a large sorting facility there and it’s the consolidation route USPS assigned the package. Another possibility is that USPS sent it there first my mistake. It’s rare, but it happens during the busy holiday shipping.