about billing though I am using free tier

Hello.

What services are listed on the invoice?
Would it be possible for you to share with us a screenshot or something?

The AWS free tier covers the usage and services described in the following documents.
For example, t2.micro and t3.micro for EC2 are free for 750 hours per month.
Any use in excess of this limit will result in billing.
https://aws.amazon.com/free/?nc1=h_ls&all-free-tier.sort-by=item.additionalFields.SortRank&all-free-tier.sort-order=asc&awsf.Free%20Tier%20Types=*all&awsf.Free%20Tier%20Categories=*all

You can search for active services in your AWS account by using the "Confirm you have terminated all active resources" method described in the following document.
Check to see if any of the services listed on the invoice are active.
https://repost.aws/knowledge-center/check-for-active-resources