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For this achievement you need to sort 30 partially addressed letters within 60 seconds in the Postmaster's Office.
The quest starts with a dropped Lost Mail on the ground near a random mailbox in Dalaran. The spawn timer for the Lost Mail seems to vary from 2 hours to 3.5 hours.
First you'll deliver the mail to Madam Goya in the Underbelly ( Lost Mail). Next you'll receive a mail from the Postmaster summoning you to the Dalaran Mailroom. This room, accessible by the pet cemetery, is crammed with magical elementals, junk lost in the mail, and even a Pepe overseeing the chaos.
Completing the Postmaster's questline will reward the The Total Package achievement. You then have access to an advanced minigame, where you must sort letters within a minute. Completing the Post Haste achievement will reward you with the the Mailemental battle pet, and you will also be mailed a Bulging Package which contains a bind when used copy of the Lost Mail (so you can give it to a friend and pay it forward), some Solid Stones, and a toy that lets you summon Katy's Stampwhistle, the mail assistant. Completing the Priority Mail achievement will reward you with the Postmaster title
Here you can buy Priority mail achievement for EU or US Servers.
Also you'll get guide for completing Priority mail achievement
• You“ll be able to get hard-to-get Priority mail achievement, save the time for yourself, get a knowledge how to get this achievement and safe your nurves as the most inmportant
• Postmaster title
• The Total Package achievement
• Mailemental battle pet