I used the first droplet in association with a Floating IP for 561 hours. The droplet was paid and the Floating IP was, thus, was free i.e. not to be charged. After deleting the Droplet, I needed to delete the Floating IP as well.
After I deletion of the first Floating IP, DigitalOcean imposed a charge $3.84 for this Floating IP for 567 hours, in addition to the above facts disregarding the situation that it was attached to the first droplet for 561 hours.
With a second droplet in association with a Floating IP for 561 hours the patern was the same. I deleted the second droplet and thereafter its associated Floating IP.
The droplet was was charged for 561 hours. The second Floating Ip was declared for free during this time and an additional charge of $0.0 was calculated in Invoice for additional 7 hours.
This shows a defective billing system because both the droplets had the same pattern: Both were used for 561 hours, both were paid. Thus, DigitalOcean should not have imposed no additional charge for both these droplets.
How is that possible that DigitalOcean does not charge for one and charges double for the second, when all other factors are the same?
This confirms a defective billing software.