I'm going through a similar issue with the Old San Juan post office. I lost my PR license, so until I can get around to replacing it, I'm using my Florida license. They will not accept my FL licence + Passport here for a PO Box. It has to be a PR license + other photo ID.
I should ask them about the cruise line crew members that I see getting their mail 3-4 times a week. What ID's did they have to show?