The Bringme Box is not designed to receive envelopes or other very small or thin items: as a result, very flat objects (thinner than 2 cm) are not detected every time.
However, anything thicker or larger than 2 cm can be delivered without any hassle.
If you still wish to place a very flat item in the box, make sure you place it right up against the side to be certain. Or if you want to place a very small item in a compartment, put it in a box first.
This will allow your delivery to be detected by the sensors every time.