Hello,

With DigitalOcean you are not billed monthly but hourly, ff you use your server for fewer than 672 hours during the month, you will be billed for each hour that you used it. If you use your server for more than 672 hours that month, you will be billed at the monthly cost.

So the charges for the first month will be billed from your referral credit, and the charges for the second month (after your referral credit expires) will be billed from your credit card.

Hope that this helps!
Regards,
Bobby