Scenario

I have tried printing a FedEx label and I am seeing an error returned:

Invalid paymentType

Solution 

This error is being returned when the Shipping Charges Payment Type is set up incorrectly.

To fix this, go to Shipping > Integrations > FedEx > Config > find the Shipping Charges Payment Type > change the value from the Account to Sender > Save.

If the error message will still be the same, feel free to contact Linnworks Technical Support for further help!