LUO YAN was walking on the streets of Goldcrest City. He was on his way to Arcadia Mail and Delivery. It's an establishment in the game where players could send mail and all kinds of items to other players. Each of the four cities had the said establishment. When he asked Shen Ji Yun for some advice on how he should deliver the invitations, he recommended the game's official mail and delivery service.
According to him, if one would want to deliver a letter to another player, they could print the letter there. All they had to do was to show the copy of the letter they wanted to print. Since the game account could be linked to a player's email address, it was easy to retrieve any file from their email even if they're inside the game. The same way that they could also sent screenshots and videos that they had taken in the game to their email.
Which meant, Luo Yan could simply show the invitation he worked so hard to make last night to any NPC there.