The main resolution die roll is what I call a diff20™. (Yes, I invented it.)
As its name suggests, it is simply the difference between two d20s.
Min result = 0.
Max result = 19.
It has a nice asymmetic probability curve: low results are more probable than greater results.
Mailed Fist does not have criticals per se, but the same effect is achieved with the diff20 itself.