Put a name on the box for easy identification, the dimensions, and the unit to be used (cm or inches) and click on the + icon in the beginning of the line.
Use a dot as the decimal separator. Like 2.1
You can also directly edit the current boxes.
Remember to Save the changes.
If you’re adding envelopes, and you want to use the packing box calculation, please make the dimensions so that thicker items will fit inside. Maybe use the maximum acceptable height of the envelope. Or you can even have several versions of different heights registered.