3. The next step is to setup your packaging on the Canada Post server. Login to the sellonline server and click on Add/Modify/Delete Boxes.
You should setup whatever envelope/packaging you are using to send via lettermail.
You can have as many as you want, but the box names MUST begin with "Lettermail".

For example, to setup number-0 bubble mailers as a packaging type, you could specify:

Description: Lettermail_0
Weight Limit: 30
Box Weight: 0.010 kg
Length: 25.4 cm
Width: 16.2 cm
Height: 2 cm

The height cannot exceed 2 cm for Lettermail rates.
How did you setup your lettermail in CP?