When you create a shipping label you select the type of envelope or package for your shipment using the Enter Packaging Details screen.
Depending on the carrier you may be prompted to enter the weight of your shipment. If you have an attached USB scale, place the package on your scale and click the scale button.
The weight will appear in the Weight field. Refer to How to create a shipping label for more information.
UPDATED: February 24, 2021