Why did you say that the extra bag you got was useless? In fact, what would be useless would be to have it when the mini world change is not active (and no bank NPC wants it). The usefulness of this bag is to get the achievements related to it. Note that it has been reported that delivering two bags at once indeed increase your bag counter by 2. This way, the only situation an extra bag would be useless would be if you already delivered 4 bags and need only one for the Goldhunter achievement. In such situation, you could sell both bags to someone who has delivered less than 4 bags, and wait for the next bank robbery to deliver your last bag.