Promocja na mąkę kończy się dziś wieczorem, więc kupuję ją teraz.

Because na here means something like on / for, as in a promotion on flour, and after na in this meaning Polish uses the accusative case.

• mąka = nominative
• mąkę = accusative

So:

• promocja na mąkę = a promotion on flour

This is a very common shop/advertising pattern in Polish:

• promocja na kawę
• promocja na masło
• promocja na chleb

Kończy się means is ending, comes to an end, or ends.

The się here does not literally mean itself in a natural English sense. It is part of a very common Polish construction where a verb becomes more intransitive or middle-voice-like.

Compare:

• kończyć coś = to finish something
• kończyć się = to end / to come to an end

So:

• Kończę pracę. = I am finishing work.
• Praca się kończy. = The work is ending.

In your sentence, promocja kończy się dziś wieczorem means the promotion ends by itself, in the sense that it reaches its end.

Wieczorem is the instrumental singular form of wieczór and is commonly used as an adverb meaning in the evening / this evening / tonight.

So:

• dziś = today
• wieczorem = in the evening

Together:

• dziś wieczorem = this evening / tonight

This is a standard Polish time expression, similar to:

• rano = in the morning
• po południu = in the afternoon
• wieczorem = in the evening
• nocą = at night

They both mean today. In most situations, the difference is very small.

• dziś is a bit shorter and often feels slightly more concise
• dzisiaj is a little fuller, but still completely normal and common

In this sentence, either would work:

• Promocja na mąkę kończy się dziś wieczorem...
• Promocja na mąkę kończy się dzisiaj wieczorem...

The first version is just a little more compact.

Kupuję is the imperfective verb form, and here it means something like I’m buying it now or I buy it now.

• kupować = imperfective, focuses on the process or ongoing action
• kupić = perfective, focuses on completion

So:

• kupuję ją teraz = I’m buying it now
• kupię ją teraz = I’ll buy it now / I’ll buy it right now

Both can make sense, but they are not exactly the same.

In your sentence, kupuję sounds natural because the speaker is reacting to the situation and presenting the action as something they are doing right now.

Ją refers to mąkę.

Since mąka is a feminine singular noun, the pronoun used for it in the accusative is ją.

So:

• mąka = flour
• kupuję mąkę = I’m buying flour
• kupuję ją = I’m buying it

This is a very normal way to avoid repeating the noun.

Because Polish pronouns agree with the grammatical gender of the noun they replace.

• mąka is feminine
• so it becomes ją in the accusative

This has nothing to do with biological gender. Even inanimate things have grammatical gender in Polish.

For example:

• chleb is masculine
• masło is neuter
• mąka is feminine

So the pronoun changes depending on the noun.

Because więc connects two clauses, and in Polish it is normally preceded by a comma.

Here the two parts are:

• Promocja na mąkę kończy się dziś wieczorem
• więc kupuję ją teraz

So the comma marks the pause and the logical connection: so / therefore.

This is standard Polish punctuation.

Więc means so, therefore, or thus.

It introduces a result or conclusion:

• the promotion ends this evening
• therefore, I’m buying it now

It is a very common everyday connector. In speech, it is extremely frequent.

Not always. In this sentence, promocja means a special offer, discounted price, or temporary deal in a shop.

So here it is closer to:

• special offer
• discount
• sale promotion

It does not mean marketing promotion in the business sense here.

Also, promocja is not always the same as wyprzedaż:

• promocja = promotion / special offer
• wyprzedaż = sale / clearance sale

Yes. Polish word order is more flexible than English word order.

Your sentence is natural and neutral:

• Promocja na mąkę kończy się dziś wieczorem, więc kupuję ją teraz.

But other versions are possible, for example:

• Dziś wieczorem kończy się promocja na mąkę, więc kupuję ją teraz.
• Promocja na mąkę kończy się dziś wieczorem, więc teraz ją kupuję.

The meaning stays similar, but the emphasis changes slightly. Polish often moves words around to highlight time, contrast, or focus.

Because mąka is usually treated like an uncountable substance noun, just like flour in English.

So:

• kupuję mąkę = I’m buying flour

You are thinking of the substance in general, not separate countable items.

Polish can use the plural mąki, but that usually means different kinds of flour:

• mąki bezglutenowe = gluten-free flours
• różne mąki = different flours

Not always. Teraz usually means now, but in real usage it can be a little flexible, just like English now.

In this sentence, it means something like:

• now, before the promotion ends
• at this moment / right away

So it does not have to mean the person is literally standing at the checkout at that exact second. It can simply mean they are acting immediately rather than waiting.