"Here you are, one fish."
Translation:Yksi kala, ole hyvä.
I feel like this one of those "that's just how it goes" things about Finnish. So I don't know if there's any particular reason for that but yes, it usually always goes at the end of a phrase. This is is what often happens in restaurants etc. and how I see that: they also make sure they're bringing to you what you ordered and when you signal to them that it was correct/your order if there is more than one person, they will then end the conversation with "ole hyvä" or such. (Or actually you're the one to end it by thanking them!)
"Ole hyvä, yksi kala." to my ear sounds maybe even a bit rude. But I also want to emphasize that if you are a non-Finn and happened to say it this way, there's still no reason to worry because no one would take it as an insult - we know that our language is difficult and sometimes weird too and we rarely take any errors with the language as an act of impoliteness :) And I wouldn't be surprised if Finns would use this as well sometimes, but most of the time it goes at the end of phrases.
I would accept both answers, but obviously as MCRmadness explained, one is more natural than the other most of the time.
Technically "ole hyvä, yksi kala" is a valid translation. It could be that the waiter/vendor handed over the fish with a "here you go", and then iterated what the purchased item was. Happens all the time.