Charges for Business Support while using a free tier account

I haven't seen your invoice so I don't know exactly what happened, but did you accidentally change to business support?
In that case, you can change from Business Support to Basic Support and no support fee will be charged from the following month.
Please change it according to the method in the following document.
https://docs.aws.amazon.com/awssupport/latest/user/changing-support-plans.html