The error can be caused by having incorrect characters (such as dots or periods) in the customs label harmonization code field as the field only accepts pure numbers. Check other fields for characters that should not be there if that is not your problem. This is absurd that paypal doens't have a javascript function to check the field for invalid characters such as dots, as checking this is trivial. It should WARN the user that they are putting bad characters into a field and not let them submit until they correct it
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