You cannot mail on behalf of the requested customer [Solution]

When processing orders, you might come across the "You cannot mail on behalf of the requested customer" error.

This error is shown when you are using one merchant's API keys (Key-Customer-Contract number) with a Customer and/or Contract number not associated to it.

3rd Party Billing is NOT supported by Canada Post. You need separate merchant API keys for each mailed on behalf account. 

• If you already registered the merchant, follow the instructions detailed here to solve the issue: Error: 2553 Paid by Customer is not a valid payer partner of the Mailed on Behalf of Customer
  
  
• If you haven't registered the merchant yet, follow the instructions detailed here and set the Billing Account on your Client accordingly to solve the issue: Registering Canada Post Account