1) Run up to the mail box
2) Zoom your camera in all the way so you're doing that crazy "First Person" view
3) You can now see the mail box because you are inside all the other players' models
4) Loot your gold from the AH ;)
This is one of my personal gripes bout WoW. Major citys do not have nuff Mailboxes. Yet the new BloodElf city has one on almost every corner... probably like 10 in the city. Shat has 5 or more that I know of.