Why am I getting charged although I deleted my instances and volumes a few weeks ago?

There are several reasons why you might still be seeing charges on your AWS account even after deleting your instances and volumes:

1. Billing lag: AWS charges for the time resources were active, even if they're no longer running. You may see charges on your bill for the current billing period for resources that were active earlier in the month.

2. Hidden resources: Some resources may not be immediately visible in the EC2 dashboard. Check for:

   • EBS snapshots that weren't deleted
   • Elastic IP addresses not associated with running instances
   • NAT gateways or other networking components

3. Regional resources: Ensure you've checked for resources across all AWS regions, not just your primary region. You may have forgotten resources in other regions.

4. Data transfer charges: You may incur charges for data transferred out of AWS to the internet or between regions and Availability Zones.

5. Free Tier limits: Even if you're using Free Tier services, there are usage limits. If you've exceeded these limits, you may be charged.

To resolve this issue:

1. Use the AWS Cost Explorer to identify specific services and regions incurring charges.

2. Check your billing dashboard and expand the Elastic Compute Cloud section to see which regions have EC2 resources.

3. Review the "Bills" section in the Billing and Cost Management Dashboard for a detailed breakdown of charges.

4. Use the AWS Support-ListEC2Resources tool to find any hidden resources.

5. Set up billing alerts to notify you when your usage approaches Free Tier limits.

If you've done all this and still can't locate the source of charges, consider opening a support case with AWS for further assistance. You can do this by going to the AWS Support Center, clicking on "Create Case," selecting "Billing" as the issue type, and providing details about your situation.
Sources
Charge for EC2 after i remove all resources | AWS re:Post
Why am I being charged money on my account? | AWS re:Post
Why have I been billed without running any instances? | AWS re:Post

It's easy to overlook that you may have spun up an EC2 in another region during your studies, you can check this using EC2 Global View https://console.aws.amazon.com/ec2globalview/home

Your screenshot of Cost Explorer shows that in Report parameters (top right) has Group by/Dimension set to none. Try setting this to Service and you might get a better idea of where these charges are coming from.

Then look at your bill in detail (this month and the previous months) to drill down and find out what individual items are incurring the charges.

Since you are using Cost Explorer, change the filters to see costs per day and then filter by region, API Operation or service. This will tell you what causing the costs. It's possible that you have enabled services like SSM Parameters or have S3 storage even have retained the EBS volumes after deleting the EC2s. Also, there are charges for VPC Nat Gateway, load balancers in case you have provisioned those.