She wrote many of the letters, several of which are thank-you notes, on her signature blue stationery.
Others include simple sketches Onassis drew for Bill Hamilton, the design director for Carolina Herrera back then, to use as a reference when designing clothes for her.
CBS sat down with Hamilton, who shared how his design process with Onassis worked.
"There were certain colors she wouldn't want to wear -- one of them being black," Hamilton said.
John F Kennedy and Jackie O through the years
Jackie Kennedy Onassis' personal letters to be auctioned
Massachusetts, USA, 1950's, President of the United States of America John F, Kennedy pictured as a young man with his wife Jackie Kennedy (Jacqueline Bouvier) early in their marriage (Photo by Popperfoto/Getty Images)
Massachusetts, USA, 1950's, President of the United States of America John F, Kennedy pictured as a young man with his wife Jackie Kennedy at home (Photo by Popperfoto/Getty Images)
NEWPORT, RI - SEPTEMBER 12: John F. Kennedy and Jacqueline Kennedy outside St. Mary's Church after their wedding on September 12, 1953. (Photo by Charles F. McCormick/The Boston Globe via Getty Images)
UNSPECIFIED - CIRCA 1754: US President John Kennedy, President Charles de Gaulle and Jackie Kennedy in Paris at the Elysee Palace May 31, 1961 (Photo by Universal History Archive/Getty Images)
1956: Waiter clearing dishes fr. table in front of actor David Niven (3R) sitting next to Sen. Jack Kennedy's wife Jackie (2R) w. others dining at society gala sponsored by wife of US Amb. to Cuba, in the ballroom of the Waldorf Astoria Hotel. At the far left is Jackie's sister Caroline Lee Bouvier (later Canfield, Radziwill, and Ross). (Photo by Yale Joel/The LIFE Picture Collection/Getty Images)
March 25, 1958: Sen. Jack Kennedy cuddling his darling baby daughter Caroline who is smiling as her mom Jackie looks on in delight while relaxing on bed at home. (Photo by Ed Clark/The LIFE Picture Collection/Getty Images)
JULY 20: John F. Kennedy and Jacqueline Kennedy. (Photo by Charles McCormick/The Boston Globe via Getty Images)
Sen. Jack Kennedy slyly lending an ear to his wife Jackie's giddy whisper as they sit together at table during cocktail hour before dining at society gala sponsored by wife of US Amb. to Cuba, at the Waldorf Astoria Hotel. (Photo by Yale Joel/The LIFE Picture Collection/Getty Images)
American future First Lady Jacqueline Kennedy (1929 - 1994) and her daughter Caroline pause on the stairs of their vacation home and look back down, Hyannisport, Massachusetts, September 1960. Ms. Kenney wears a trenchcoat and a flower-print scarf over her hair while Caroline is dressed in a overalls. (Photo by Alfred Eisenstaedt/The LIFE Picture Collection/Getty Images)
Jackie Kennedy and friends, circa 1960s. (Photo by Fotosearch/Getty Images).
CIRCA 1960s: Former First Lady Jacqueline Kennedy enjoys herself at a picnic circa the 1960s. (Photo by Michael Ochs Archives/Getty Images)
DISTRICT OF COLUMBIA, UNITED STATES - 1961: President Kennedy (R) with First Lady Jackie (L) (in fur-trimmed suit designed by Oleg Cassini) at his inauguration. (Photo by Leonard McCombe/Life Magazine/The LIFE Picture Collection/Getty Images)
PARIS, FRANCE - UNSPECIFIED DATE: American president John and Jackie Kennedy at the American Embassy during 1961 in Paris, France. (Photo by S010/COLLECTION SALINGER/Gamma-Rapho via Getty Images)
The newly-elected President John Fitzgerald Kennedy with wife Jackie after giving the traditional acceptance speech. USA, November 1960 (Photo by Mondadori Portfolio via Getty Images)
Frank Sinatra escorting Jacqueline Kennedy to her box at a gala, held at the National Guard Armory in Washington DC, the night before the inauguration of President John F. Kennedy, 19th January 1961. The gala was staged by Sinatra to help pay off the campaign debts of Kennedy and the Democratic Party. (Photo by GAB Archive/Redferns)
Portrait of American First Lady Jacqueline Kennedy (1960 - 1999) in the Red Room of the White House, Washington DC, January 15, 1962. During the filming of a CBS News Special program called 'A Tour of the White House with Mrs. John F. Kennedy,' she stands in front of an Alfred Wordsworth Thompson painting of the Civil War battle at Balls Bluff, Virginia. (Photo by CBS Photo Archive/Getty Images)
The President and Mrs. Kennedy attend a White House Ceremony February 19, 1963. (Photo by National Archive/Newsmakers)
First Lady Jackie Kennedy speaks to a guest while attending a White House ceremony January 8, 1963 in Washington, DC. (Photo by National Archive/Newsmakers)
UNITED STATES : A contemporary report ran: ?Caroline, five, moves into the big time. Big day for a small girl. The day she moved from the junior to the senior class. The day she rode not her own pony, Macaroni, but mother?s horse, Rufus, with her mother. The girl is Caroline Kennedy, five-year-old daughter of America?s President John Kennedy and his wife Jacqueline.? President Kennedy was assassinated later that year, on November 22. (Photo by SSPL/Getty Images)
18th October 1968: Millionaire shipping magnate Aristotle Onassis (1906 - 1975) with his wife Jackie (Bouvier Kennedy, 1929 - 1994). (Photo by Central Press/Getty Images)
Jackie Kennedy Onassis, Caroline Kennedy (left), John F. Kennedy, Jr., and Lee Rodziwill (Photo by Ron Galella/WireImage)
Jackie Onassis, Anthony Radziwill, and Tina Radziwill (Photo by Ron Galella/WireImage)
Jackie Onassis during Jackie Kennedy and Family Shopping in Capri - August 24, 1970 in Capri, Italy. (Photo by Ron Galella/WireImage)
Jackie Kennedy Onassis in Capri, Italy. (Photo by Ron Galella/WireImage)
Jackie Onassis (Photo by Ron Galella/WireImage)
Jackie Onassis with son John Kennedy Jr during Jackie Onassis Sighting at the Philharmonic Hall - December 13, 1971 at The Philharmonic Hall in New York City, United States. (Photo by Tom Wargacki/WireImage)
Washington, D.C. May 14th 1976. Jackie Onassis and Alexandrje Orfila at The Kennedy Center (Photo by Tom Wargacki/WireImage)
Jackie Onassis and Alex Orfila during OAS Party following the Kennedy Center Gala at Kennedy Center in Washington D.C., United States. (Photo by Ron Galella/WireImage)
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"Onassis wrote she was 'so sick of everyone constantly in black -- like Mediterranean villages where everyone is in mourning for 20 years,'" CBS correspondent Vicente Arenas read from one of Onassis' letters.
The correspondence also shows Onassis had strong opinions about pant legs, jacket shapes and the use of shoulder pads in her clothing.
Onassis' interior decorator told CBS she was exactly the same way about what went into her house. She made all the final decisions, even things as small as what sort of trim went on the pillows.
The co-owner of the auction company believes these letters show a different side of Onassis.
"In these notes you get a sense of how someone famous can make the average person comfortable. She really did go out of her way to make people feel appreciated," Rico Baca told the Orlando Sentinel.
The letters, along with some photos of Onassis, have been separated into 30 lots for the auction. Baca believes bidding for each will start between $800 and $1,200.