I had one trip last fall (around Halloween) that didn’t get charged to my credit cards. Those sessions also never showed up on my Tesla account. As far as Tesla was concerned, they never happened. It happened over a week-long trip across multiple states.
In all other cases, by the time I get seated in the car and put it in gear after disconnecting, my phone chimes with an alert from the credit card company that the authorization has come through. This shows the same dollar value that is shown on the charging status screen in the car.
when it works, it’s instant. If not instant, it seems to be free (even without a promotion that would make it free).