From Eden Eternal Wiki
The Bank (also known as Warehouse) is found in Aven and acts as extra storage. You can access it through NPC GG Bank Seth or GG Bank Never. It's a great place to keep items that you don't need now, but can use later.
Much like your own backpack, the warehouse space can be expanded by adding additional bags. However, before you can equip another bag, you must first rent out an additional warehouse slot to place the bag in. These bank slot rentals can be purchased in the item mall.
Note: Bank slot rentals are effective for a month after use, meaning items can be placed and taken from the extra slots for that month. After the month is up, items cannot be placed, only removed from the extra slots.