I have the exact same question. The variables country of origin and customs code can be added to the products in shopify. But the fields are useless if these fields are not in the Liquid variables list and there is no information anywhere on the code needed to add this variable to the invoice template. The liquid variables list in shopify should be updated to add these 2 variables, there is no reason for it not to be on the list as it is a standard variable. It just seems like these 2 variables missing on the Liquid variables list is a bug/error in Shopify that should be fixed