vnat

Hi guys, This shouldn't be complicated but for some reason not seeing an easy way to do this. I want to offer a service free for 30 days. If the user cancels within the 30-days they cancel the plan and are not charged. If they don't do anything, they get billed on the 30th day (for the 2nd month as the 1st month was free) Can anyone advise of how to configure this please? Thank you