There's a series of quests in Sunrise Hills that result in Featherweight Satchel of the Courier, a 20-slot, 100% weight reduction bag. There are also some additional rewards, like a clicky illusion and a placeable figure.
The first part of the quest involves Postmaster Aric Songfairer. The quest is described in the comments section on allah's, scroll down after following the link.
The next part is from Pagus Nonad, who is the Tax Collector in Sunrise that usually is doing some tradeskills. Again, the quest is described in the link.
After completing these quests, hail Lysric Loresinger for your new bag!
Lysric and Aric are usually around the stage behind the first building in sunrise. You might have to wait a bit if they're performing.
20-slot Bag Quest
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