Though the grammatically correct answer is "To all", you will find that some native writers will write "To All"...the reason for this is usually to emphasize that it is to "All" and not just to the one person who may first receive the letter.
Where are you using it on the greeting card?
As a note inside the card? Use "To all"
Or on the outside of the envelope? Use "To All"
Hope this helps!
Student or Learner