A letter can only be cancelled if your cancellation window has not yet elapsed. For all accounts, a default cancellation window of 5 minutes is set for letters. Furthermore, accounts with a Growth Edition subscription have the ability to modify their cancellation windows to be set between 1 minutes to 3 days. If you'd like the ability to modify your cancellation window, check out our Growth edition.
Once the cancellation window for a letter has elapsed, the letter will not be able to be cancelled.
You can cancel a letter one of two ways: