Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Sorry to hear you're having trouble.
However, as this is an open forum I'm afraid that no one here would be able to provide you with any answers about letters being sent. You could try contacting support via https://support.microsoft.com and click on Contact Us then follow the instructions to start an online chat but even they may not be able to tell you if mail has been received or not yet.
Please accept my apologies for this.