Custom hidden field
 

When i add a new product is there a way to enter a word or number so only the admin will see it on the invoice?

Wouldn't your customer also see it when they print the invoice, or when you (the admin) prints the invoice to include with the order?

You could hide it with a usertype statement.

http://forum.x-cart.com/viewtopic.php?t=13502&highlight=usertype

That's a sample. Admin is usertype == A.