At the moment there is a confusion with the way billing information is shown in history.
If there are payments - they are negative numbers. I.e. paid 5$ and it shows as -5$ payment.
If there are referral reward added i.e. 25$, then they are also shown as -25$ in the history.
If you started with DROPLET10 promo code - you have -10$ shown for the history.
I would suggest that you fix the "-" signs before numbers as is standard with accounting worldwide. If the balance is added to - the sign should be "+", or no sign. If the balance is substracted from - the sign should be "-". That way it will be more clear what was happening in the past history.
Also, due to DigitalOcean subtracting payment per minute, but actually charging it later on on the monthly basis - you are issueing invoices. Those invoices should be shown with negative number.