Providing your logistics partner Canada Post the IOSS number correctly ensures your shipments are handled appropriately.
Commercial customers will have the capability to electronically transmit an IOSS number to the receiving post in the EU when using one of the following Canada Post shipping systems:
EST Desktop and EST Online:
Use the ‘Tax ID/IRS/VAT’ field.
ONLY enter the IOSS number in the following format- IMxxxxxxxxxx
SnapShip and Shipping Manager:
Under Step 5 ‘Customs’ section, enter IOSS number in the ‘Tax ID/IRS number’ field.
ONLY enter the IOSS number in the following format- IMxxxxxxxxxx
Note: Commercial customers will not be able to transmit the IOSS number if creating a label at any Retail post office. They will need to use one of the shipping systems above and then drop the package off at a Retail post office.
You can read more at Canada Post website: Canada Post IOSS Instructions