Sotheby's had estimated that the letters would sell for $250,000 to $350,000 at Friday's sale.
The cartoonist wrote the affectionate letters to a young woman in 1970-1971.
The 44 letters included 22 original drawings of some of the comic strip's characters, including Charlie Brown, Snoopy and Lucy.
Sotheby's called it the most significant collection of correspondence and drawings by Schulz to come to auction.
Schulz died in 2000 at age 77.
More Nation & World Headlines
Al-Qaida captures leader of U.S.-backed Syrian rebels California cuts water use by 27 percent George McGovern revealed he had a secret child Pentagon OKs guns to protect stateside troops Man describes finding 777 part on Reunion Island Blue moon? Celestial event comes with a catch U.S. looking for lion-killing Minnesota dentist 3 hurt in emergency evacuation of Boeing MD-80
Our new comment system is not supported in IE 7. Please upgrade your browser here.