I received this reply last month from Prince Edward & Countess Sophie. I had written to them to express my condolences following the death of Edward's father, Prince Philip.
Other than the black edge for mourning, the envelope is the usual from the Wessexes. You may notice their mail does not feature the usual postmark that we see on other British Royals' mail. This is because the Wessexes's mail is not processed through the Court Post Office at Buckingham Palace, which processes all the mail from the London-based Royal offices. Instead, the Wessexes's mail is handled by their staff at their home at Bagshot Park in Surrey.
This postmark features their Sovereign Grant's old PPI number of W 7047. This was not redesigned in 2019 like the other royal postmarks to coincide with the switch to W 4047. You can find more the different Royal Postmarks here.
The back of the envelope features Prince Edward & Countess Sophie's joint coat of arms, above "Bagshot Park" the name of their home where the mail was sent from.
"In memory of His Royal Highness Prince Philip, Duke of Edinburgh 1921-2021"
It’s with great sadness that we acknowledge the death of our Patron HRH The Duke of Edinburgh, he will be sadly missed. pic.twitter.com/27Ui8HFQAu— DofE (@DofE) April 9, 2021
Choosing a Duke of Edinburgh Awards Event would be particularly meaningful to the Wessexes as they are heavily involved with the organization.
The card says:
Thank you for your kind message of condolence. We are extremely grateful for your thoughtfulness in writing at this sad time.
TRH The Earl and Countess of Wessex and Forfar