"Here you are, a coffee."
Translation:Kahvi, ole hyvä.
The -ko/kö is something that can be added to any word to make it a question. So "kahviko" is a question word that could be used in questions where the coffee has the emphasis. For example, "Kahviko on sinun lempijuomasi?" = "Is it coffee that is your favourite drink?". The question "Onko kahvi lempijuomasi?" = "Is coffee your favourite drink?" would also be constructed using -ko but it has a different emphasis.
Yksi kahvi, kaksi kahvia, vähän kahvia, lisää kahvia, for some examples.
"Haluatko kahvin?" is asking specifically if you want a/one coffee (e.g. do you want to stop at this gas station to grab a cup on the go) while "haluatko kahvia?" asks if you want some coffee (e.g. you're visiting someone's home) - the amount isn't specified.
The word is always kahvi for coffee, even if you use a different inflected form of it. We've loaned the usual Italian words/phrases for espresso-based stuff too, of course.
You can say it either "kahvi, ole hyvä" or "ole hyvä, kahvi", that order doesn't matter. "ole hyvä" is used for "here you are" when giving something, and for "you're welcome" in response to someone thanking you. In the case of giving something, I guess it's a sort of pre-emptive "you're welcome", to which the recipient answers "kiitos".
It's part of the idiom "ole hyvä", the equivalent of "you're welcome", or in this context, e.g. "here you are". Much as the English phrase is literally wishing someone welcome (to a place?) despite actually being used in response to someone thanking you, the Finnish phrase literally means "be good", but is used idiomatically for something a bit different.