Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
I'll help you.
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It's very easy to find out where the charge is coming from, once you know how to do it. Everything that you purchase directly from Microsoft goes through your Microsoft account. Your Microsoft account is making the purchase, not you, and your Microsoft account is paying for that purchase with the payment method that you stored in that Microsoft account.
Here is your Microsoft account: https://account.microsoft.com/account. Sign in to your Microsoft account with its username and password. Once you have signed in, go to the account's Order history.
Order history shows you what that Microsoft account has purchased. If you have more than one Microsoft account, do the same thing with each Microsoft account in turn. If you see something that costs $16.53 a month (which may include sales tax) then you have your answer. If you don't see something costing $16.53 a month, then you know that your Microsoft accounts(s) did not make that purchase.
If your Microsoft accounts did not make that purchase, that means someone used your payment information to buy something from Microsoft in your name. If you don't know anyone who might do that, perhaps a family member, that means someone unknown to you has stolen your payment information and is using it to buy stuff. Today from Microsoft, tomorrow? Contact your card issuer or bank ASAP and tell them that an unauthorized person used your account to make an unauthorized purchase.
Microsoft can't help you here. Someone paid for something with valid payment information so the sale is valid. Microsoft can't see who's tapping on the keyboard, and it isn't Microsoft's responsibility to determine whether someone entering payment information is actually entitled to use that information.