How to setup an Elastic IP that is free of charge?

Hello, I'm trying to setup an Elastic IP on my EC2 instance with the objective of having a static IP and I'm following these two articles:

(1) https://repost.aws/questions/QUUC7uf_inRZeiTVh5AwpPyQ#COoZoHOySnS9OpC2H5bYuzeQ

(2) https://repost.aws/knowledge-center/elastic-ip-charges

But I still have some doubts.

In the first place, I'm being charged for the service, I can see this in my cost explorer. I used an Elastic IP for a few days and my cost this month grew up 624% related to my usual cost per month.

On the othar hand, according to the article on the knowledge center (2), if I have the following setup, it should be free of charge:

• The Elastic IP address is associated with an EC2 instance.
  • I associated the Elastic IP with my only already running instance.
• The instance associated with the Elastic IP address is running.
  • The instance is already running.
• The instance has only one Elastic IP address attached to it.
  • The instance has only one Elastic IP attached to it, I have only one Elastic IP.
• The Elastic IP address is associated with an attached network interface. For more information, see Network interface basics.
  • The Elastic IP is associated with the instance, if I associate it with the interface it will disassociate with the instance, which is the first requirement.

But, however, I don't exactly understand how can I have the Elastic IP associated with the instance and with the interface at the same time. I can only associate the Elastic IP with one resource, the instance OR the network interface, but I can't have it associated with both.

Also, I see that the configuration summary of the Elastic IP is the same no matter if I associate it with the instance or with the network interface.

The network interface that I associate the Elastic IP with, is the same interface that is associated with the instance in question.

Finally, I am also aware of the fact that from the 1st of february this year (2024), there IS some charge per hour for the public IPv4, as this article points out.

So in conclusion,

• Am I doing this wrong?
• How to correctly associate the Elastic IP (do I associate it with the instance or with the interface)?
• And finally, if I have the instance running, and I have associated the Elastic IP with the network interface, why am I being charged?

Thanks in advance for any insight on the question!

PS/PD: Here is a snapshot of one day (11/03/24) of my cost and usage graph as reference for the question in the comments. Which one should be representing the cost of the Elastic IP?