The Smash invitation is an envelope stamped in wax with the Super Smash Bros. logo featured in promotional materials and character reveal trailers for Super Smash Bros. 4 and Super Smash Bros. Ultimate. It first appeared in Villager's reveal trailer for Super Smash Bros. 4 and has since appeared in multiple other Super Smash Bros. Ultimate character trailers.
The invitation first made an appearance in Villager's trailer. In it, he is seen taking the envelope out of his mailbox before running off. Interestingly, this is the only appearance of the Smash invitation in any Smash 4 character trailer. Due to this, it is likely based on the letters one can receive in the Animal Crossing series.
The Smash invitation is much more prevalent in promotion for Ultimate characters, especially within the trailers for DLC characters. It first appeared in Isabelle's trailer, in which Pete (a pelican mailman from the Animal Crossing universe) delivers one to her. Later, in Joker's first trailer, he is seen running through The Game Awards 2018 while his teammates comment on what he may be there for. As it's shortly revealed, he, in fact, arrived to steal a Smash invitation. It reappears at the beginning of Joker's second trailer. Following that, it had made an appearance in Terry Bogard's reveal trailer, parodying the Bernstein clan's invitations seen throughout The King of Fighters series. In it, multiple characters from The King of Fighters series attempt to grab it as it flies through the air: Kyo Kusanagi, Ryo Sakazaki, Andy Bogard, Joe Higashi, Geese Howard, Nakoruru via her hawk Mamahaha, and Iori Yagami. It finally lands on the ground, allowing Terry to take it. Most recently, it appeared in Byleth's reveal trailer, in which Sothis summoned it to gift to the aforementioned fighter.
Aside from character reveal trailers, the Smash invitation has also been used as a general motif for the game's downloadable content. This can be seen in the Japanese download card for the Fighters Pass Vol. 1 and Vol. 2, which features an image of a Smash invitation on its front. It is often used in relation to unrevealed downloadable characters, as can be seen in several Super Smash Bros. Ultimate presentations and information on the game's website pertaining to downloadable content. It was also used in the September 4, 2019 Nintendo Direct to represent the confirmation of additional downloadable fighters outside of the Fighters Pass Vol. 1, months later it was confirmed that there will be six more playable characters in the Fighters Pass Vol. 2, and each challenger pack makes use of the smash invitation as a placeholder since the characters have yet to be revealed.