Use this form to request Form 6166, a letter of U.S. residency certification for purposes of claiming benefits under an income tax treaty or value added tax (VAT) exemption. You cannot use Form 6166 to substantiate that U.S. taxes were paid for purposes of claiming a foreign tax credit.
Clarification on mailing address for the Form 8802 -- Feb-10-17
Other Items You May Find Useful
Publication 519, U.S. Tax Guide for Aliens.
Publication 947, Practice Before the IRS and Power of Attorney.