So we know that Susie draws on a punch card after Sans asks us to. Coincidentally, in Undertale, we can go through some stuff at Sans's house to see a card with 3 people drawn on it with the text "Don't forget".
Well, that means this and that are the same thing from different worlds, case closed right? But hold it... If we tell Ralsei that we'll buy him ice cream in chapter 3, Susie will draw 4 people: Kris, Susie, Noelle and Ralsei", as we're getting ice cream for Ralsei as well. But if we are not going to promise to buy him ice cream, Suise will draw only 3 people as she dosen't remember about Ralsei. But... Still, this makes no sense. Why would simple ice cream have such an impact?
... That's because it's not about Ice cream at all
In the weird route, when we go to the ice cream stand, Sans will give the punch card again. But Susie will draw only 3 people: Kris, Susie and Ralsei. That's because she's not as close to Noelle as in the normal route. That means, what happens in Undertale is actually after the Weird Route, promoting Sans and Papyrus to move out to Undertale. And whatever happened made Sans write "Don't forget" on the Punch Card that's tied to the fun gang