According to a letter from Congressman Tancredo, but mailed by the U.S Immigration Reform PAC (USIRP) -- not mailed at government expense, "to elect more people to Congress committed to ending mass immigration" contributions can be sent to:
USIRP
P.O. Box 12024
Arlington, Virginia 22219
And just to be complete, the usual note on the bottom of the contribution card reads: "Individuals can contribute a maximum of $5,000 to a political action committee each year. Contributions are NOT tax-deductible for income tax purposes. Federal law requires that the occupation and employer be reported for individuals giving a total of $200 or more in one calendar year. Checks should be made payable to USIRP".
No phone number or web site given in the letter.