Error: 2553 Paid by Customer is not a valid payer partner of the Mailed on Behalf of Customer [Solution]

When processing an order, you might come across the "Error: 2553 Paid by Customer is not a valid payer partner of the Mailed on Behalf of Customer". 

NOTE: This error applies to Canada Post orders only.

This error is related either to:

• Using the same API key for different shippers and payers. Remember that Canada Post does NOT support 3rd party billing. 
  For each pair of API Keys, there is ONE Paid by Customer # and Contract ID.
  If you are trying to use a new Paid by Customer #, you will have to Register a new Merchant as described here (Register a new Canada Post Merchant) and then fill up the fields as described below.
  
  
  
• Billing Account (Carrier) misconfiguration. If this is the case make sure you have the right credentials before making any changes.

Feel free to follow the step-by-step guide written below or the video guide, as you prefer:

To solve this issue you will have to:

1. Access Clients under the Maintenance menu
   
    
   
   
2. Click on the client related to the order
   
   
3. Edit the Billing Account related to the order and error
   
    
4. Select the Carrier Options tab
   
   
5. Verify that the information entered in the Paid by Customer # and Contract ID fields match with your registration information with the carrier
   
   
   
6. (Optional) If you made changes, press Save