Need Account # on Estimates and Invoices

I'm using Invoice, not the Creator database Zoho Invoice.

Is it just me or does anyone else want to display the customer's Account Number on Estimates and Invoices? When customers call with billing questions, one of the first thing we ask is "What is your account number"? And it would be a lot easier to answer that question if the number was on the invoice. Every invoice I receive in the mail has my Account Number on it.

It looks like estimates and invoices are created for Customers, not Accounts. So the Account table is not part of the documents and the account fields are not in the field list.

Maybe I'm missing something. Is there any way to insert the Account Number on an Invoice?

Thank you in advance for your consideration.
DJ