Singed by the Duchess herself, the thank you note also featured a rare photo of the bride and groom on their wedding day, which has never been released before.
“Thank you so much for your amazingly caring and supportive messages for Beatrice and Edo’s future happiness,” the Duchess of York wrote.
"It was a glorious day; the sun smiled down on them. We were all delighted to be able to come together and celebrate their love for one another.
"Beatrice looked stunning and Edo handsome – their love shone through. We are so happy to welcome Edo into our family and thank him for his love and devotion to Beatrice."
Fergie concluded by thanking the fan for many years of kindness and loyalty.
Other royal fans were quick to comment on the post, with one person writing: “Wow so kind of her to send a never seen before photo !!!! Now I cannot wait for mine to come.”
Another person stated: “What a beautiful card from Sarah, she really knows how to send the sweetest thank yous.”
A third person added: “Wonderful and thank you for sharing.”
Beatrice and Edo's surprise wedding was an intimate affair, which took place at the Royal Chapel of All Saints at Royal Lodge on Friday July 17.
The bride herself has also been sending out thank you cards, with a photo of one card recently appearing on the Duchess of Wonderland Instagram account.
In the card, Beatrice wrote: "Thank you so much for the wonderful note you so kindly sent to us on the occasion of our wedding this summer at Royal Lodge. Edo and I had a magical day filled with special moments with our families.
“We are so grateful to all those who have worked hard to protect our communities over the last few months, we owe them so much.
"Thank you for thinking of us as we begin our married life. Beatrice and Edo," she added.