Question: "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

> 
> Wasser bitte.
> 
> 
> 

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.

Answer: 
These are not at all long for a native German. Any of these are fine depending on circumstances. If you want to use the last sentence when buying cigarettes in a shop you would say:

> 
> Ich hätte gern eine Schachtel Zigaretten.
> 
> 
> 

, otherwise the way you wrote it, you'd inform somebody that you enjoy buying a package of cigarettes (e.g. after somebody asked you to do so).

Now, what would a native say? In a restaurant if you order just *Wasser*, you'd get a glass of water. So if you want a bottle of water you have to specify that.

> 
> Eine Flasche Wasser bitte. 
> 
> 
> 

is perfectly fine, but likely you also want to specify *mit/ohne Kohlensäure (stilles)*.

I don't smoke, but from what I saw/heard, people would usually use the brand name to refer to a package of cigarettes. So you could say:

> 
> Eine Schachtel [brand name] bitte.
> 
> 
>
The quantifier allows to differentiate between specific and non-specific amounts. If the amount is important, e.g. when shopping, use a quantifier:

> 
> Ich kaufe *eine Flasche* Wasser
> 
> 
> I buy *one bottle* of water (not more, not less)
> 
> 
> 

If you want to make a general statement, that is not related to a specific amount, you do not quantify:

> 
> Ich mag *Zigaretten*
> 
> 
> I like *cigarettes* (in general)
> 
> 
> 

Any further additions (like *bitte* or *gerne* in your example) are part of constructions to convey politeness, but not required for the sentence per se.