Have you also looked at the excellent website cruisecritic that will give you a ton of info about the ports you will visit.
www.boards.cruisecritic.com - and go to the Ports of Call section.
I may have been wrong about the Post Office - it would be worth trying them, Go into one - there's nothing helpful on line which is what misled me.
You will have to order in advance - normally for next day delivery if you go in early enough in the day.