You can do Sam Seabones' quest as many times as you like, but you have to log off between them. The same for Josie McReedy and Johnny McVane (land quests), and Bartholomew Watkins (sailing).
To find a zip code for any U.S. city, go to
www.usps.gov and it will give you the info. Then you can send that letter to your American friend (wink).