Also, I definitely have to second this: If it feels fishy, back out.

If you hear the words “entirely commission-based”, back out. If your employer mentions that they have done any stealing before and you’re not 300% sure that you’ll get the money (ergo it’s already in the hands of an escrow service you trust, or you already have it), back out.

Lastly, your estimate is probably wrong. Learn to accept that–try to identify why it goes wrong and by how much, and you’ll start learning to make it more accurate.

Companies want someone with your expertise in their corner. People want someone to come to with questions. Contracting is a people business. Yes, it’s important the customer understands what is going to happen. Yes, it’s important to get contracts signed and half the money up front. But it’s also important to fill the customer’s needs (however fluid they might be).

People (at least some of them) are willing to pay for service. You can find those people by telling them how much it is going to cost up front, and explaining that you do development in an iterative way. Then, you solve their problem. Businesses don’t just buy random code, they have a problem they want solved. If you solve that problem, your contract is a success. Most businessmen are willing to pay a good price for a service that completely solves their problem. They know their problem is in flux; they know that they may not have the solution at the time of the beginning of the project. They know they might find a better solution during development. They want that eventuality planned in. Tell them you are planning for change, and tell them that planning for it demands an iterative approach, and if they want a fixed price, it costs more because of the extra uncertainty. Explain the benefits of the system.