Finnish does not have the indefinite article "a," or, for that matter, any articles. "A coffee, please" would be translated as "Kahvi, kiitos." "Yksi" translates to "one," never "a."
When translating from Finnish to English, it is sometimes necessary to infer from the Finnish word order whether the English translation should use "a" or "the" before a noun. If I understand it correctly, words at the beginning of a Finnish sentence are more likely to be translated into English with "the," and toward the end of a Finnish sentence with "a."
"Kahviko" is a question, "kahvi" is a noun. The -ko, -kö endings are used when forming questions, like: "Tämä on" - this is "Onko tämä?" - is this? "Otat aurinkoa" - you take sun (meaning: you are sunbathing) "Otatko aurinkoa?" - are you taking sun? "Vie tämä pois" - take this away "Vietkö tämän pois?" - Could you take this away?
Note that this is also a polite way to ask someone to do something, as we tend to express ourselves very bluntly.