Eu vou ao correio para enviar o pacote.

Because vou is the first-person singular present tense of the verb ir (to go).

• ir = the infinitive, meaning to go
• eu vou = I go / I am going

So:

• Eu vou ao correio = I’m going to the post office

Portuguese usually uses the present tense very often for actions happening now or in the near future, so eu vou can sound like I’m going.

Yes, it can often be omitted.

Portuguese is a pro-drop language, which means the subject pronoun is often unnecessary because the verb ending already shows who the subject is.

• Eu vou ao correio = correct
• Vou ao correio = also correct and very natural

The pronoun eu may be included for:

• emphasis
• contrast
• clarity

For example:

• Eu vou ao correio, mas ele fica em casa.
  I’m going to the post office, but he’s staying home.

Ao is a contraction of:

• a = to
• o = the

So:

• a + o = ao

That is why:

• vou ao correio = I’m going to the post office

This contraction is required in standard Portuguese.

Some related contractions:

• a + a = à
• a + os = aos
• a + as = às

Because Portuguese often uses the definite article where English may not.

Here, correio means the post office or the mail service, and the sentence is using it as a destination:

• ao correio = to the post office

In Portuguese, it is very normal to say:

• vou ao banco = I’m going to the bank
• vou ao supermercado = I’m going to the supermarket
• vou à escola = I’m going to school

So the article is part of the natural structure.

In this sentence, correio most naturally means post office.

The context makes that clear:

• Eu vou ao correio para enviar o pacote.
  You go somewhere physical in order to send a package.

So ao correio here is best understood as to the post office.

Depending on context, correio can also relate to mail/post, but in this sentence it refers to the place.

Para here expresses purpose: in order to / to.

So:

• para enviar o pacote = to send the package / in order to send the package

This is a very common structure in Portuguese:

• Estudo para aprender. = I study to learn.
• Fui ao mercado para comprar pão. = I went to the market to buy bread.

So the full sentence means:

• I’m going to the post office in order to send the package.

Because after para expressing purpose, Portuguese commonly uses the infinitive.

So:

• para enviar = to send
• para comprar = to buy
• para estudar = to study

You are not saying I send; you are saying the purpose of going is to send.

Yes. In Brazilian Portuguese, pra is a very common spoken contraction of para.

So both are understood:

• para enviar o pacote = more neutral/full form
• pra enviar o pacote = very common in speech

In writing, especially more formal writing, para is usually preferred. In everyday conversation, pra sounds very natural.

Because Portuguese often uses the definite article when referring to a specific thing already understood in context.

• o pacote = the package

This suggests a particular package, not just any package.

Compare:

• enviar o pacote = send the package
• enviar um pacote = send a package

So if the English meaning is the package, then o pacote is exactly right.

You may hear similar patterns in informal Brazilian speech with some places, but vou ao correio is the standard and safest choice.

Why?

• ir a = the standard verb-preposition combination for to go to
• a + o = ao

So:

• Vou ao correio = standard, recommended

In casual spoken Brazilian Portuguese, some speakers say ir em / no with locations, especially in everyday speech, but for a learner, ao correio is the best form to use.

Grammatically, vou is present tense, but in context it can express a current action or a near future.

So Eu vou ao correio can mean:

• I’m going to the post office
• I go to the post office
• sometimes even I’m going to go to the post office, depending on context

Most naturally here, it sounds like:

• I’m going to the post office to send the package

A learner-friendly approximation is:

• coh-HEY-oh

A few notes:

• the rr in Brazilian Portuguese is not like the English r
• in many accents, it sounds like a breathy h sound
• the stress falls on rei

So correio sounds roughly like coh-HEI-u.

Yes, it is perfectly natural and clear.

It sounds like a normal everyday sentence meaning someone is going to the post office for a specific purpose.

Very natural variations include:

• Vou ao correio enviar o pacote.
• Eu vou aos Correios para enviar o pacote.

Depending on region and context, people may also refer to the Brazilian postal service as os Correios. But the original sentence is absolutely correct and natural.