I suggest you call her and tell her you rechecked your old notes and found that you had only quoted her the $150. You could explain how you normally charge $xxx, but were trying to give the city a first time discount. Offer to return the extra money or apply it as a deposit for the next booking. My guess is you'll get to keep the money, lighten your conscience and get another booking at the price you really want to charge.

It's a gamble, but the odds are in your favor; and it shows you in a good light.