I agree with TimtheYoutuber on this point, if the agreement was basically to pay $300 for the work then that is the contract between you no matter how hard the work is, if you think this work is worth more you should have asked for this from the beginning.
It had been specifically communicated to them mid-work about the scope of the work being changed and they agreed to it.
Anyhow, how did you miss the whole point of not even paying me the 300 that was promised in the "contract"? Please read the thread again. This isn't about extra 200 being demanded but not even paying the initial 300 as agreed.