Alongside the message, there was a black-and-white image of young Charles and Philip riding in a small motorboat at Cowes, Isle of Wight, from 1958.
"The Prince of Wales thanks you so much for your very kind message of sympathy," the card read. "His Royal Highness has been enormously touched by the many generous messages that have been received in recent days; they have provided great comfort at this very sad time."
The note concluded: "The Prince of Wales sends you his warmest thanks and best wishes."
The sentimental letter was bordered by black lining, which is traditionally used by members of the royal family when in mourning.
The change-up was perhaps in honor of Philip, who famously had a no-fuss attitude about life and death. Philip designed every detail of his own funeral — with his coffin being placed on the back of a Land Rover — opting for no eulogy at the somber occasion.
Charles was joined by his mother, Prince William, Kate Middleton, Prince Harry and other close royal family members for Philip's Saturday, April 17, funeral at St. George's Chapel at Windsor Castle. Charles' other daughter-in-law, Meghan Markle, was not in attendance, as her doctor advised her not to fly due to her pregnancy.
After Philip's death, the 72-year-old shared a touching message with the public, praising the 99-year-old for being an incredible man and the public for their outpouring of love and support.
"My dear Papa was a very special person who I think above all else would have been amazed by the reaction and the touching things that have been said about him and from that point of view we are, my family, deeply grateful for all that," Charles said. "It will sustain us in this particular loss and at this particularly sad time."