Image on the carte de visite is 2 1/8 x 3 1/4. Issued by Anthony's New York studio, but from a negative from Brady. Hunter stands, hand on hip, in a somewhat traditional politician's pose. The free frank signed envelope is from some time in his service in Washington prior to the Civil War.
The envelope is empty-contents long gone. I have not seen the particular cancellation before with "FREE" part of the stamp. What I have seen in the past is "FREE" in a straight line often under the signature. Robert Mercer Taliaferro Hunter (1809-87) was a US Representative 1837-43 & 1845-7 & Speaker of the House 1839-41.
He served in the Senate from 1847-61. He then served in the Confederate Senate from 1862-5 & was a fierce critic of Jefferson Davis. Post war he was Treasurer of Virginia 1874-80. Nice pair & would look good framed.
His signature is somewhat uncommon.