The IRS address in Kansas City (Zip Code 64999) is a legitimate processing center, and receiving Letter 12C is just a request for manual verification of details your e-file couldn't automatically confirm, often due to a mismatch in reported withholding or estimated tax payments as compared to what they expected.
To resolve this, you should send the requested documents, including the IRS letter to one of the methods provided. If you prefer to mail them to the Missouri address, I suggest you use Certified Mail with a Return Receipt for your records, as this provides legal proof of delivery while you wait for the IRS to finish processing your 2025 return. Faxing, however, will be cheaper and provide you with an instant proof of submission.
