You don't provide much in the way of context so let me add some so I can go into more detail than theDBGuy did. You are also talking about printing a form which you should not be doing. Reports are optimized for printing. Forms are not.
On your Invoice form, you have a combo that allows you to pick a customer. The combo shows the customer but the CustomerID is what is saved to the table. Now, you want to create an Invoice report so you bind the main report to the Order table but all it has is the CustomerID and you want to show the name and billing address on the invoice. The solution is to create a query that joins the order table to the customer table on CustomerID and selects the relevant customer fields from the customer table as well as the relevant order fields from the order table. Then you use this new query as the RecordSource for the Invoice.