If this error comes up when shipping internationally with FedEx, this usually means that one or more line items in the customs declarations might have an invalid character. You might also see a similar error for your second, third, or fourth customs line item. (Please note that if you’re seeing an “Incomplete commodity description” error, you will need to visit this article instead.)
What you need to do when this comes up, is to go and edit your customs declarations and replace any special characters you might have added. For example, the percent sign (%) can be replaced with the word ‘percent’.