What I've just done is go to Customer, Edit Customer   (Our customers are homeowners, so no company names).
First Name  =   John

Last Name   =  Smith

Customer Display Name  =   Smith, John

Name to Print on Checks =  John Smith

When I test an invoice, the invoice is addressed to John Smith.  

All customers are now alpha sorted by the Customer Display Name, which is the Last Name.  It's beautiful!