Thanks so much for the question, here is how our billing works:
At the end of each calendar month, we calculate the cost of your server based on the size of your droplet. All droplets are billed at an hourly rate up to a maximum monthly cap, which we have set at 672 hours (28 days).
Therefore, if you use the server for less than the monthly cap, you will be billed for each hour that you used it. However, if you use your server for more than 672 hours during that month, you will be billed at the monthly cost, which will be no greater than the cost of 672 hours.
Additional services (automatic backups, etc.) may lead to further charges.
Any payments made with PayPal add credit to your balance, we ask that you maintain a credit on your account balance equal to the estimated monthly charge of your droplets if you only use PayPal.
If you have added a credit card to your account, any due balances will be billed automatically on the first of the month. Should there be any issues with charging your card for the month we will notify you via email.
If you have promotional credits as well, then promotional credits will be utilized first and then credits applied from PayPal will be used before charging any remaining balance to the credit card.
Hope this helps!