The stamp is for the 44th Monte Carlo International Circus Festival which took place on January 16th-26th. Each year, the Monaco Post releases a new stamp in celebration of the festival. (Princess Stephanie is President of the Festival.)
The postmark is for the Museum of Prehistoric Anthropology in Monaco. This postmark ran the week of July 13th-18th, ran again in August & will run again in September. This is one of those postmarks that is used multiple times a year when there isn't a special postmark.
The back of the envelope features Prince Albert's monogram.
Inside is a flat card. This card is the same as Prince Albert's past birthday replies, except the message has been changed and the date updated.
The monogram is embossed. And the signature is printed on. The date is Prince Albert's birthday.
The card says:
Palace of MonacoMarch 14th, 2020.
I was very grateful to receive your kind wishes on the occasion of my Birthday and I send you my very best regards.
Albert de Monaco