Overage tasks are charged at a rate of 1.25 times the plan's task cost. For example, the “Professional” level of Zapier is $89/mo for 5,000 tasks per month.
The equation for overage tasks is as follows:
$89 / 5,000 = 0.0178
$0.0178 x 1.25 = $0.02225 per overage task.
For more info, see: How pay-per-task billing works
Pay-per-task can be enabled or disabled in the Billing and usage page. Once updated they will need to click on Update your plan.