How much should i charge to make an ecommerce website. Please help, urgent.?

A client of mine owns a large electronic store. He wants me to make him an eCommerce website with a shopping cart, payout system, complex databases, and all of the bells and whistles. We agreed on a flat fee and i said that i would get back to him tomorrow with a price. I do not want to charge per hour because i feel that it is almost cheating. Like, what if i spend an hour of a four hour project going to eat. No one is monitoring me. I do not think that is fair. I know how to make this website in depth, but i am 13, and this is a big deal for me for my future. How much should i charge for New York standards? How much should i deduct because of my lack of real world experience? Please help!