Żwirek kosztował dziś trzydzieści złotych, więc kupiliśmy dwa mniejsze worki zamiast jednego dużego.

No. It is only capitalized because it is the first word in the sentence. Normally it would be written żwirek.

As a noun, żwirek is a diminutive form of żwir. Depending on context, it can mean small gravel or cat litter. In everyday Polish, diminutives are very common and often sound natural rather than especially cute.

Kosztował is the past tense of kosztować (to cost).

It has the ending used for a masculine singular subject, because żwirek is a masculine noun.

So:

• żwirek kosztował = the litter/gravel cost
• if the subject were neuter, you would get kosztowało
• if it were feminine, kosztowała

Polish word order is much more flexible than English word order. Dziś means today, and it can appear in different places depending on emphasis and style.

For example, all of these are possible:

• Żwirek kosztował dziś trzydzieści złotych
• Dziś żwirek kosztował trzydzieści złotych
• Żwirek dziś kosztował trzydzieści złotych

The version in your sentence sounds natural and neutral.

After numerals 5 and higher, Polish normally uses the genitive plural form of the counted noun.

That is why you get:

• jeden złoty
• dwa / trzy / cztery złote
• pięć złotych
• trzydzieści złotych

So złotych is the correct form after trzydzieści.

Więc means so, therefore, or as a result.

It connects the two parts of the sentence:

• the price was high
• so we bought something else

So the structure is:

• Żwirek kosztował dziś trzydzieści złotych, więc...
• The litter cost thirty zlotys today, so...

It is a very common everyday connector.

Because kupiliśmy is perfective, and it presents the action as completed: we bought.

Compare:

• kupiliśmy = we bought / we ended up buying
• kupowaliśmy = we were buying / we used to buy / we bought repeatedly

In this sentence, the idea is a single finished choice: because the price was high, we bought two smaller bags.

In the Polish past tense, first person plural forms include both:

• a past-tense stem
• a personal ending meaning we

So:

• kupiliśmy = we bought

The -liśmy form is used when the group is masculine-personal or mixed. If the speakers were an all-female group, it would be kupiłyśmy.

So this form tells you not only we, but also something about the grammatical gender type of the group.

Because dwa (two) combines with this noun in the normal 2–4 pattern.

Worek is a masculine inanimate noun, and in this structure you get:

• dwa worki = two bags
• dwa mniejsze worki = two smaller bags

The adjective also agrees with the noun:

• dwa = two
• mniejsze = smaller
• worki = bags

Since worki is inanimate, its accusative plural looks the same as its nominative plural, which is why you see worki here.

Because mniejsze means smaller, not just small.

It is the comparative form of mały (small):

• mały = small
• mniejszy / mniejsze = smaller

The sentence contrasts:

• dwa mniejsze worki = two smaller bags
• jednego dużego = one big one

So the comparative makes sense: they bought smaller bags instead of one large bag.

Because Polish often omits a noun when it is obvious from context.

Here, worka (bag) is understood, so the full version would be:

• zamiast jednego dużego worka

But since worka is easy to infer from dwa mniejsze worki, Polish naturally leaves it out.

English does something similar:

• two smaller bags instead of one big one

So jednego dużego really means one big bag.

Because zamiast requires the genitive case.

So:

• jeden duży worek = nominative
• zamiast jednego dużego worka = genitive after zamiast

Even when worka is omitted, the adjectives and numeral still stay in the genitive:

• zamiast jednego dużego

That is why you do not see jeden duży here.

Yes. Polish word order is flexible, and different orders can sound more neutral, more emphatic, or more literary.

For example, you could also say:

• Dziś żwirek kosztował trzydzieści złotych, więc kupiliśmy dwa mniejsze worki zamiast jednego dużego.
• Żwirek dziś kosztował trzydzieści złotych, więc kupiliśmy dwa mniejsze worki zamiast jednego dużego.

The original version is perfectly natural. Changing the order would mainly change emphasis, not the basic meaning.