[IgorPartola]

I do not generally insist that people sign my contracts. This is a bad sign. The logic of "each task should take no more than 2 hours is flawed". You can easily combat this by breaking up tasks further into subcategories. In fact, that is generally a good idea for tasks over 8-16 hours when creating estimates. So there are things you can do on your end.

Having said that, I would give them a call and explain politely that you are a professional and your work is worth what you are billing them. If they are not going to pay you, then you will simply stop working for them (assuming you can afford to do so). You can even say "if you cannot afford to have me work X hours, that is one thing. Let's talk about that. But otherwise initially you agreed to pay X for Y. I delivered the work, and you are not paying. Why should I continue working with you?"