"Buying something" is just to put it in a special context so it's easy for you to think back.
I'm new to German so it's quite struggling for me to buy something in terms of specifying the quantity. For example, I would say, as translated from English:
Eine Flasche Wasser bitte.
instead of just
because it sounds kind of awkward when you think in English term, normally we ask for "a bottle of water" rather than just "I wanna buy (some) water". But then, the former way sounds kind of long and I'm not sure if native speakers bother to say that many syllables.
Same thing for buying cigarette in the Supermarket for example, how do you say it naturally? I would normally say
Und ich kaufe gern eine Schachtel Zigaretten
But it sounds kind of long again, I've seen native speakers mumbling something very brief and the Kassiererin still managed to understand lol.