You must customize your sales invoice form.  Since "Custom Fields" are not available to select to appear in  the body of an invoice form, the field you want to add is "Customer Part Number" as a column, under field types-Other Fields.  Then for every item you want to have the master item number printed in the invoice under the column labeled "Customer Part Number" you must go to the maintenance record of that item and add the master item number in the field "Part Number".  If that represents too much manual work for you, I can quote you a price to globally change your item records.  Call Kim 800 303-7770