Я уже купила всё для ужина, но потом пришлось докупить ещё сыр и доплатить за пакет.

Because the speaker is female.

In the past tense, Russian verbs agree with gender and number:

• я купил = I bought (male speaker)
• я купила = I bought (female speaker)
• мы купили = we bought

So Я уже купила... tells you that the speaker is a woman.

Уже means already.

So:

• Я уже купила всё для ужина = I had already bought everything for dinner

It shows that this action was completed before the next part of the story.

Because всё means everything, while все means everyone or all people / all things depending on context.

Compare:

• всё = everything
• все = everyone / all

Here the sentence means I bought everything for dinner, so всё is the correct form.

Для requires the genitive case.

So:

• ужин = dinner
• для ужина = for dinner

This is a very common pattern:

• для друга = for a friend
• для работы = for work
• для семьи = for the family

So всё для ужина literally means everything for dinner.

Пришлось comes from прийтись and is often used in an impersonal construction meaning had to.

So:

• мне пришлось уйти = I had to leave
• ему пришлось ждать = he had to wait

In your sentence, мне is omitted because it is obvious from context:

• Я уже купила всё для ужина, но потом пришлось...
• full version: ...но потом мне пришлось...

So it means but then I had to...

The form пришлось is neuter singular because impersonal past-tense constructions in Russian usually use this form.

Because the speaker has already been established as я, so Russian often leaves out things that are easy to understand from context.

Full version:

• Я уже купила всё для ужина, но потом мне пришлось докупить...

Natural shorter version:

• Я уже купила всё для ужина, но потом пришлось докупить...

This is common in spoken and informal written Russian.

Купить means to buy.

Докупить means to buy some more, to buy additionally, or to finish buying what was missing.

So the idea is:

• first, she bought everything she thought she needed
• later, she realized she still needed something else
• so she докупила / had to докупить more items

In this sentence, докупить ещё сыр means to buy some more cheese / to additionally buy cheese.

The prefix до- often adds the idea of adding, finishing, or bringing something up to what is needed.

Платить = to pay
Доплатить = to pay extra / to add an extra payment

So:

• доплатить за пакет = to pay extra for a bag

This suggests that the speaker had already been paying for the groceries, but then an extra charge was added for the bag.

Because сыр is the direct object of докупить, so it is in the accusative case.

For an inanimate masculine noun like сыр, the accusative looks the same as the nominative:

• nominative: сыр
• accusative: сыр

So:

• докупить сыр = to buy cheese
• докупить ещё сыр = to buy more cheese / some additional cheese

You may also sometimes hear сыра, especially when the speaker means some cheese in a more indefinite or partitive sense, but сыр is completely normal here.

Because платить за means to pay for something.

This pattern uses за + accusative:

• платить за кофе = to pay for coffee
• платить за билет = to pay for a ticket
• доплатить за пакет = to pay extra for a bag

Here:

• пакет is masculine inanimate
• accusative = пакет

So за пакет is correct.

Specifically, it suggests an extra payment.

• платить за пакет = pay for a bag
• доплатить за пакет = pay extra for a bag / pay an additional amount for a bag

So the sentence implies that the bag was an extra cost not included in the original purchase.

Because the sentence describes completed, single actions.

• купила = bought, completed
• докупить = buy additionally, as a completed action
• доплатить = pay extra, as a completed action

Russian often uses the perfective aspect for events in a sequence:

• first she bought everything
• then she had to buy more cheese
• then she had to pay extra for the bag

If you used imperfective forms, the meaning would change and would sound more like process, repetition, or general activity rather than single completed events.

Russian word order is flexible, but this order is very natural.

• потом = then / afterwards
• пришлось = had to

So но потом пришлось... means but then I had to...

Placing потом early helps show the sequence of events clearly:

1. Я уже купила всё для ужина
2. но потом пришлось...

This highlights the contrast: she thought she was done, but later something else came up.

Но means but and introduces a contrast.

The contrast is:

• I had already bought everything for dinner
• but then I still had to buy more cheese and pay extra for a bag

So но shows that the second part goes against the expectation created by the first part.

Yes. You could say:

• Я уже купила всё для ужина, но потом мне пришлось докупить ещё сыр и доплатить за пакет.

This is slightly fuller and more explicit.

The version without мне is more compact and very natural when the subject is already obvious.

Yes.

After пришлось, you get infinitives describing what the speaker had to do:

• докупить
• доплатить

So the structure is:

• пришлось + infinitive + и + infinitive

In other words:

• had to buy more cheese and pay extra for a bag

This is a very common pattern in Russian.