The Bringme Box is not designed to receive envelopes or other very small or thin items in the parcel compartments. As a result, very flat objects (thinner than 2 cm) are not detected every time by the sensors.
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 Bringme Box, make sure you place it up against the side of the compartment to be certain. Or if you want to place a very small item in a compartment, put it in a container first. This will allow your delivery to be detected by the sensors every time.