I asked this question and made the suggestion to add this feature some years ago as well. Understand the instructions here are a clunky, manually entered, workaround. If you do this as we have your estimates will have one terms field and invoices will have two terms fields. If you make a mistake with the manually entered terms then these two terms fields will not agree with each other and your customers will be confused. Intuit, give us the option of including the linked terms field on the estimate already!