It depends on how magical the concept is. If someone asks me for an ecommerce site, I'm fairly certain I can deliver it, my client has seen dozens of ecommerce

It depends on how magical the concept is. If someone asks me for an ecommerce site, I'm fairly certain I can deliver it, my client has seen dozens of ecommerce sites so they are pretty certain I can deliver it. Payment upfront is just a formality at this point.