What is AutoPay?

http://ddgrow.com/best-ptc-list

What is AutoPay?
If you have AutoPay enabled, on the first click a referral makes per day, you'll pay to keep that referral for an extra day.
If the referral does not click, you won't pay him for another day.

So, imagine you rented 500 referrals by spending $100 for a month's rent.
Instead of paying $100 again at the next due date (yes, it hurts), you can pay for them daily.
In the end, you'll save $10 and you'll never run out of referrals (*).
Also, you don't have to worry about the due date (*).
While it's easier to pay this way, it's also cheaper for you because you save 10% of a whole month's rent.

Basically, you'll pay one day in advance to your referral's due date.

The value for each referral/day will be deducted from your rental balance.
If your rental balance has insufficient funds, it will deduct from your main balance.
If both balances have insufficient funds, it won't be triggered and will only work again when one of the balances has enough funds.

(*) Please keep in mind that while you may have AutoPay activated, it's only triggered for referrals that have 20 or more days left until next month's payment. This value is smaller for some Golden Packs