I received this reply earlier this year. I had written to Crown Prince Frederik for his birthday in Mary.
The envelope is the usual from the couple.
The card is identical to past Birthday replies from the Prince. The couple use the same flat cards each year, so they can make the in bulk, save money and not have any waste year to year. That is why the card does not have any specific mention to the year or age of the Prince.
Every few years, when they have to print of new cards, they do slightly change their card. You can see the past evolutions here.
The card says:
HRH Crown Princess & Crown Princess's Office
Amalienborg Palace
His Royal Highness The Crown Prince has asked me to thank you for your kind congratulations on the occasion of his birthday.
Frederik VIII Palace
Amalienborg 1257 Copenhagen
