The front of the envelope is pretty usual for the Belgium Royals. They have they have a lot of different ones that they use, but this one is pretty common. The front features a Palace of Brussels underneath.
Inside is a typed card & a photo of Queen Mathilde.
The photo was taken at Castle of Laeken, the King & Queen's residence. The [Royal] Palace of Brussels is a working palace where they have their offices.
The card is the usual from Queen Mathilde.