On the meantime, the King of England once, appalled, called his assistant because an envelope fell to the ground next to his desktop and he didn’t know what to do

On one occasion he rang me from his library and he said 'Oh, um, Paul, a letter from the Queen seems to have

The anecdote you referenced appears to be from Paul Burrell, the former royal butler to Princess Diana and Queen Elizabeth II. In his memoirs and public interviews, Burrell has shared several behind-the-scenes stories about the royal family, including moments involving King Charles III (then Prince Charles). The specific story about Charles calling his assistant because a letter from the Queen fell to the ground and he didn’t know what to do is mentioned in Burrell’s accounts, particularly in his book "A Royal Duty" and in subsequent media appearances ​