I Completed Big Walk's Final, Best-Hidden Secret Puzzle, And The Reward Was Delightful

A URL appears at the end of Big Walk's credits, and following the website's instructions leads to a physical reward mailed to the player: a red shape covered in multicolored confetti, the same kind given for every other puzzle in the game. The author found the wrinkled envelope while sorting vacation mail, and it contained a small bonus as well. The offer is only valid "while supplies last."