Sachet

This item comes free with a purchase of an additional bag slot. When a new bag slot is purchased, the Sachet appears in that new slot automatically as a baseline storage (no bag slot can be empty). The Sachet in a slot can be swapped with a better bag by dragging the new bag into that slot. The swapped Sachet will appear in your inventory. If you have an extra Sachet you don't need, you can sell it to NPC vendors to somehow compensate the price of a new bag slot.
 * Note: Always keep one extra Sachet just in case you want to swap bags with your alt, you won't be able to do that without an extra bag!