My Storage is "Gigantique"

Definetely the best name for an achievement. Ever.

Sold by Haris Pilton in Worlds End Tavern in Shattrath, Lower City for 1200 gold.

Well, the achievement is to "Equip" the bag. Have you tried unequipping it and re-equipping it? Perhaps that's what's needed. If not, I'd say this is a bug that is likely to get fixed. A lot of players would complain if the actual objective is to buy one. Edit I don't really understand why people rate this comment down. Noone has answered whether it's helpful or not, and it's not something just anyone can verify. If it doesn't detect bags when reequipping, then sure, I'll delete. But until someone confirms one or the other, it stays.