Linked Extra-Dimensional Space Boxes

Linked extra-dimensional space boxes, or LEDS boxes as they are also called, are based off of magic items such as a bag of holding or a portable hole but are unique in the way that you don't have to carry them with you in order to access the content somewhere else as long as there is a LEDS box available there. Although an old magic technology first invented by mages who wished to send each other items quickly, LEDS boxes are now used primarily by banks and postal services.

Utility

As is evident from the name, linked extra-dimensional space boxes are containers that store an extra-dimensional space. However, instead of being like a bag of holding where it is always the same extra-dimensional space that can be accessed within, LEDS boxes can be used for several that correspond to keys or tokens of the users.
For example, if you go to a bank in one city and choose to have a safety deposit box in their LEDS box service, they will open up a new extra-dimensional space for you in a box or even a vault of the size you choose and they link that space to a token of some sort that is the only one that can open that particular space. They can also create a copy of the token that is kept in a safe in the bank in case you should lose yours. Then if you go to a bank in another city with access to LEDS technology you are allowed into the room with LEDS boxes of the corresponding size and can unlock your extra-dimensional space using the token, finding the items that you stored in the first city inside.
LEDS boxes are also used by some postal services where getting mail across large distances quickly is of great importance. In those cases each post office has tokens that open up the same extra-dimensional space.

Manufacturing

The magic is dependent on the boxes being the same size so they have to be meticulously measured and crafted. The magic of linking the extra-dimensional spaces to the tokens and having them open only when the token touches a box of the right size is an involved process and cannot be rushed. Any mistakes could have consequences such as opening a portal to another plane and sucking people in before closing permanently.

Access & Availability

Most banks in Arbressian have at least one size of LEDS boxes but it is an expensive service that is only really accessible to the rich. 
However, the postal systems with access to LEDS boxes are often less costly as it significantly cuts the expenses of paying mail carriers to cross great distances in order to deliver the mail and thus offsets the cost of paying for the LEDS boxes.