Exclusive discounts like our family plan, any recurring promo code added at the time of activation, or any service credit added to your line requires AutoPay to be ON.
AutoPay charges your credit or debit card 2 days before your billing cycle ends, and the next month's plan starts. For example, if your plan expires on the 30th, AutoPay will charge your saved payment method on the 28th. You should also keep in mind that your AutoPay will be automatically switched OFF if you change your credit card.
You will still receive an invoice via email detailing all charges. You can see how to enable and disable AutoPay here.