How do I cancel the Landing Page add-on?

The Landing page add-on is included in PRO and Max plans, you cannot disable it.

To stop the charges for the Landing Page in Email plan go to My account>>My billing>>Add-ons and disable the auto-renewal at least two days before the renewal date. You will be able to enable it any time you want in the future.

The ToS of the Landing page add-on is here.