Isn't therapy there to be able to heal and not to forget? The last session should mean that the client is not traumatised again.
The expectation of retaliation for not paying is a perfect setup. The decline of the card could be intentional, just for the client to realize in a deeply emotional situation that the dark memory doesn't hurt anymore. The wide eyes, I want to believe that it comes from the bewilderment that the familiar, expected pain is gone.