You can set a monthly spend limit on each card attached to your Letter account and designate how it refills.
To create budget limits, log in to your Letter account, go to your checking accounts, and tap on the checking account and card you wish to set a budget for. You will see an option about half way down for setting a budget for the card, and the refill frequency. Save the limit you choose and you'll be ready to go. If you need to update the limit, use the same process to change or remove the limit you set.