Also being sold is the Queen’s Gallantry Medal awarded for the Second World War veteran’s “outstanding courage” in helping to stop Anne, then 23, from being grabbed on The Mall in March 1974.
Mr Callender was driving the Princess and her then-husband Capt Mark Phillips when Ian Ball swerved his Ford Escort in front, forcing the Daimler to a halt.
Ball shot police bodyguard James Beaton in the shoulder and put a gun to Mr Callender’s head.
As labourer Ball, 26, tried to pull Anne out, Mr Callender grabbed his arm but was shot in the chest.
Passers-by journalist Brian McConnell and PC Michael Hills were also shot before ex-boxer Ronald Russell ran over and punched Ball in the head, ending the kidnap bid. Ball has since been detained in Broadmoor high-security psychiatric hospital.
Edinburgh-born Mr Callender and the other wounded men survived. He died 20 years ago and his children are auctioning his attack archive, which may fetch £30,000.
Charles sent the two-page letter within days while he was serving aboard HMS Jupiter in the Pacific. He wrote: “I was absolutely appalled to hear of what you had to go through…it sounded like the most ghastly nightmare…I do hope you are almost fully recovered.”
Telegrams of thanks sent by the Royal Family are included in the lot. The Princess wrote after his medal award: “We are so pleased and you thoroughly deserve it. Anne.”
Mr Callender’s war medals make up the archive which is being sold by London firm Spink’s on November 29. Marcus Budgen, head of its medals department, said: “He thought nothing of taking a bullet to protect Princess Anne. A true hero.”
Mr Callender drove the Royals for nearly 30 years, including taking Prince Charles and Princess Diana to their 1981 honeymoon.
