Of course we'd be sad to see you go, but here's a quick guide to closing your Tumult account:
Cancelling your account must be done by your account's primary contact. (See here for how to change your account's primary contact).
Start by accessing the 'Admin' menu via the bubble with your initials in the header:
Then, select the "Bill Payment" tab, and click "Deactivate Account".
You will see a confirmation popup, and you can then close your account with Tumult.