RDS Free Tier Charge

You've identified it correctly, only gp2 volumes are in-scope of Free Tier for RDS https://aws.amazon.com/rds/free/

• 20 GB of General Purpose SSD (gp2) storage per month.

You should be able to convert youir gp3 to gp2 on-the-fly, I can't find an AWS page that explicitly says this but it was discussed at length in the answers to this question https://repost.aws/questions/QUDPKCzJclQbCwOt47lf7lFQ/changing-rds-storage-from-gp2-to-gp3

I agree that it's annoying that the setting defaults to gp3. The AWS re:Post community does not have visibility of your account, nor any billing information associated with it. To query any charges, including asking for your charges for gp3 to be waived, it is best to log a billing support call with AWS https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/billing-get-answers.html#billing-support

The charge you're seeing is likely due to using GP3 storage instead of GP2 for your RDS instance. The AWS RDS Free Tier specifically includes 20 GB of General Purpose SSD (gp2) storage per month. GP3 storage is not part of the free tier offering.

When setting up an RDS instance, even if you select the free tier option, it's possible that GP3 was selected as the default storage type. This could be because GP3 is a newer and more flexible storage option that AWS may be promoting. However, to stay within the free tier, you should manually select GP2 storage during the RDS instance creation process.

To resolve this issue and ensure you're using the free tier benefits:

1. Verify that your instance is using a supported free tier instance class (db.t3.micro or db.t4g.micro for PostgreSQL).
2. Check if you're within the 750 hours per month limit for the free tier.
3. Modify your RDS instance to use GP2 storage instead of GP3.
4. Ensure you're not exceeding the 20 GB storage limit for the free tier.

If you've made these adjustments and still see unexpected charges, it would be advisable to open a support ticket with AWS to clarify the situation. They can provide specific guidance based on your account and usage details.

Remember that the free tier is designed to help you get started, but it's always important to carefully review the settings when creating resources to ensure they align with the free tier specifications.

Sources
Can some one confirm why RDS free tier is charged a small amount? | AWS re:Post
Do I currently have free tier / will this be covered by free tier | AWS re:Post
Hosted PostgreSQL - Amazon RDS for PostgreSQL - AWS