If you find you need to completely cancel your debit card, you can do so in your Letter account.

Sign in to Letter and navigate to the checking account the card is attached to. In the checking account, tap on the cards tab and select the card you wish to cancel.  Tap on Settings and choose Cancel Card. Confirm this is what you want to do, and your card will be permanently canceled. To order a new card, choose Create Card from your cards tab and you can choose to create either a new physical or new virtual card.