We are aware that some players have not received the "A Wishful Invitation" ticket even after completing all the listed requirements.

If you have already finished the quests shown in the mailbox:

"The Dream Castle" (Merlin)
"Friendship Is Everything" (Merlin)
"Making Cents of Things" (Scrooge McDuck)
"Scrooge McDuck's Grand Re-opening" (Scrooge McDuck)
"Fishing Expedition" (Goofy)

...but still haven't received the "A Wishful Invitation" ticket, please also check Remy's progression.
You need to unlock Remy and complete his "A Restaurant Makeover" quest for the ticket to appear.