Why is Free tier RDS Sql single instance usage showing monthly estimated bill??

Hello.

As stated in the document below, "db.t3.micro" should be included in the free tier, so there is no problem.
Also, the estimate displayed at the time of creation is the fee that will be charged when the free tier expires, so there is no problem if you create an RDS instance with the specifications of the free tier.
https://aws.amazon.com/rds/free/?nc1=h_ls

750 hours of Amazon RDS Single-AZ db.t3.micro instance usage running SQL Server Express Edition each month.

I agree with everything that @Riku_Kobayashi says in his answer, as long as your account is still in-scope of Free Tier, which essentially means it is under 12 months old, and all your usage is within scope of Free Tier then you shouldn't be charged anything when your invoice is sent at the end of the month.

https://console.aws.amazon.com/billing/home#/freetier will show what your Free Tier entitlements are, and how much you have used of each so far.

Ensure that you're using an instance type in-scope of Free Tier (db.t3.micro or db.t4g.micro), single-AZ only, a database engine that is in-scope of Free Tier (MySQL, MariaDB, PostgreSQL or SQL Server Express Edition) and no more than 20GB of disk.

As well as the link from @Riku_Kobayashi also see https://aws.amazon.com/rds/pricing/#Free_tier

If your invoice comes in and you feel you have been billed for something unfairly, you can log a billing support case about it https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/billing-get-answers.html#billing-support