"– A coffee? – And a pulla, please."
Translation:– Kahviko? – Ja pulla, kiitos.
"Kahviko" implies that it is a (cup of) coffee, not just any amount of coffee. That's the way you might say it if you are working in a coffee shop and making sure the customer wanted one cup of coffee. "Kahvia" would be when you have guests and you offer them some coffee.
Well if you respond "and a pulla, please" you are implying that you also want the coffee so you get the yes/no answer indirectly. And yeah, "Kahvia?" does sound more natural than "kahviko?"... the latter sounds like a confirmation question. You know, like asking if you heard the other person right. Don't know if my response clears anything up lol
I think the "ja" implies yes to coffee in the answer.
I'm not a native Finnish speaker but I've been learning the language for a while now and I think both "kahviko" and "kahvia" would be acceptable ways to phrase the question in certain contexts. But Duolingo is probably using "kahviko" in order to teach us how to use "-ko".
Yeah...these are these short, very common sentences that are almost more like grunts. Each person in the exchange is so aware of the limited options to be expressed, that the sentences aren't always formed according to proper grammar.
But since the English specifies "A" coffee, I think the Finnish has to be Kahvi(ko).
A more general question using just the word Coffee with a question mark or rising intonation would indeed be the partitive Kahvia(ko).