Credits are then available to buy in packs of 500 credits that must be paid for in advance, using a credit card. Individual packs of credits can be purchased on an ad hoc basis, but for uninterrupted campaign sending we recommend enabling SMS auto renewal under Plans & Billing in the Account Settings. When this setting is enabled and a valid payment card is added to the account, you will automatically be charged for a new pack of 500 credits whenever your balance dips below 50 credits.