To make it short: Nói (the main character) get into the coffee of the local gas station (where Iris works as a waitress) and asks for something to drink. Normal – so far.
Then Iris asks Noi if he wants to drink it in the coffee or take it away. Again, this is really common in places such as Starbucks and similar.
The surprise is that in this case the juice is actually cheaper if the customer drinks it in.
Subverting the “eat in / take away” balance has an economic reason: the juice bottle has a cost, and drinking it inside the coffee means the possibility to have it back.
But it also has a social reason, or at least a social consequence: it invites people drinking inside. The rule becomes one of the factors to spark a bit of “social life” into the bar (much needed, if you work at the coffee of a gas station in a remote fishing village in western Iceland).
Fostering participation is a difficult stuff. It involves environment design, a consistent conception of every touchpoint, a content to share, a great work on identity and trust, but also strong business rules: everything should row in the same direction. That’s the experience design, I guess.