The front of the envelope is the usual.
The back of the envelope features a crown with the address for Amalienborg written underneath. This is the envelope the couple has been using for years.
Inside is a flat card. This is the same card I've received for a number of Crown Prince Frederik's past birthday. But, what is interesting is that the couple redesigned their cards in 2017. And this card features their old style. The reason is for things like birthdays, The Crown Prince Couple print off a bunch of cards that can be used for multiple years. And, so this is just a leftover. They will switch over to the new design next time they need more.
However, Frederik's Birthday Reply last year did feature the new design, as they printed off a unique card for his 50th birthday.
The cards says:
His Royal Highness The Crown Prince has asked me to thank you for your kind congratulations on the occasion of his birthdayCheif of the Court of Their Royal Highnesses The Crown Prince and Crown Princess of Denmark