Geez Laura....you certainly have my sympathies. I read your post and thought 'there but for the grace of God go I'. As a sole practitioner with no other office assistance I live in dread of situations like this.
Make it up to them the best way you know how. Let them know, without excuses, that you messed up. Thank them even for helping you to identify a flaw in your system of Quality Control. Were I in your situation, I would amend the 2006 for free and offer 2007 either free or at the lowest rate you can afford. I might even send a gift with a formal apology.
In the meantime, forgive yourself your humanity. We are all vulnerable to mistakes, especially those of us who fly with no net.