There is no IRS Department of Advocacy Services. The IRS Tax Advocate Service (TAS) is a legitimate department of the IRS -- however, it never initiates first contact with a taxpayer. The taxpayer must initiate first contact with TAS.
So, while I won't say that the organization who sent you the letter is a scam -- I can say that the organization is not the IRS -- it may be simily an organization seeking to provide you with debt consolidation services (for a price, of course).
In my opinion, the only legitimate debt relief service is a licensed bankruptcy lawyer in your jurisdiction. Typical debt relief agencies charge huge commissions, whereas consumer bankruptcy attorneys have generally fixed charges imposed by the federal court system. Bankruptcy is more effective, and generally less costly.
If you are concerned about the letter, you can complain to the USPS Poetal Inspection Service.
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