"Do you have any milkshake or ice cream?"
Translation:Onko teillä pirtelöä tai jäätelöä?
"Haluatko pirtelöä tai jäätelöä?" - milkshake or ice cream, in the sense that the asker is also allowing you to choose both if you want
"Haluatko pirtelöä vai jäätelöä?" - milkshake or ice cream, in the sense that you have to choose only one of these options, no exceptions
"Onko teillä pirtelöä vai jäätelöä?" is a slightly odd question without context because the asker would be asking it knowing they have only "pirtelö" or "jäätelö", absolutely not both, they are 100 % certain.
And so the "Onko teillä pirtelöä tai jäätelöä?" question is the more neutral question because it doesn't assume anything. Maybe they have neither of these, maybe only the one sort, maybe both.
However, I can't remember what kinds of "tai" and "vai" sentences they have had here on Duolingo...