Personally I think if an artist cancels a commission, they should refund the full amount. When an artist takes on a commission they should only have two choices, either finish it or refund the money and say, "Noep. Not gon' do it." In the end, if said commissioner was that much of a pain, you might as well make an A_B post on him.

If the commissioner cancels the commission, then at that point it should be up to the artist's ToS.

These are of course just my opinions.