A gift card box. These could be marketed in different colors and different lid patterns. They could be sold in plastic bags on the same racks where gift cards are displayed.
In most of mine, I have used flat tiles on the top, but of course, other LEGO elements could be used, with the card being hidden in the base.
This box is 9x13 studs, as the most common gift card (or credit card) is 7x11 studs.
Unfortunately, there is no 9x13 plate available. Were such a plate produced, the number of layers in the box could be dropped. The simplest would be a VERY minimalist 3 layers: 9x13 base, 10 1x4 plates for the middle layer, and another 9x13 for the lid (without any decoration).
With decoration on top, the minimum would be 4 layers.
WITHOUT a 9x13 plate, 5 layers seems to work best, as the double layer top and bottom provide structural integrity.
Added Section 4
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This image shows a version of the box with side tabs, making it easier to open.
I also have a couple of other versions, including one with a bit "missing" in the corner of the middle layer, again for ease of opening.
In my opinion, these are best built with studs all around the middle layer, so that the box cannot open by accident.
(These versions are not shown due to the 5 picture limit, but I would be happy to send them to CUUSOO upon request.)