If the receipt says $120 and it's not specified on that receipt as a "down payment" until the work is done and a balance of the $90 is due then you've lost $90. Maybe the customer will realize once they see the receipt that it wasnt the $210 amount and you'll get the balance due.

I would suggest having your hubby handle the bookkeeping etc. You concentrate on the artistic part. When you have a potential customer have a form made up with all the important information pertaining to the job order and along with that is a quote sheet with what you have verbally told the client, have them initial it thus you have a binding receipt. Then give all these receipts to hubby who in turn makes up the invoices for you to give to people when the job in done. The stress of dealing with numbers is eliminated