Unrecognized charges

Do you have any details or results from Cost Explorer or Cost and Usage Reports? Without these we won't be able to answer this question.

Further this is the Amazon Polly Free Tier specs. If you passed 12 months it would no longer apply.

For Amazon Polly’s Standard voices, the free tier includes 5 million characters per month for speech or Speech Marks requests, for the first 12 months, starting from your first request for speech. For Neural voices, the free tier includes 1 million characters per month for speech or Speech Marks requests, for the first 12 months, starting from your first request for speech. For Long-Form voices, the free tier includes 500 thousand characters per month for speech or Speech Marks requests, for the first 12 months, starting from your first request for speech.