This field is useful to us when shipping internationally, important for Customs.  Please advise if there is a field than can be used to then print on sales invoice.  Would using one of the additional fields work? 

If I recall correctly, the code does not change (though I think the US has a slightly different code for some things than Canada does).  You could use the Inventory and Services Additional Info fields as you suggest and put them on the invoice.  This could be tested on a copy of your original invoice and a copy of your original data before you use it live.