Assim que a entrega chegar, eu vou abrir o pacote na cozinha.

Because after expressions like assim que, Portuguese usually uses the future subjunctive when talking about something that has not happened yet.

So Assim que a entrega chegar... means as soon as the delivery arrives...

This can be confusing because with many regular verbs, the future subjunctive looks exactly like the infinitive. That is what happens with chegar.

Here, chegar is not an infinitive in meaning. It is the verb form required after assim que for a future event.

Compare:

Assim que a entrega chegar, eu vou abrir o pacote.
future event

Quando a entrega chega, eu abro o pacote.
habitual/repeated situation

Eu vou abrir is the very common ir + infinitive future, which is extremely natural in spoken Brazilian Portuguese.

So:

eu vou abrir = everyday spoken style
abrirei = more formal, more written, sometimes more literary

Both are grammatically correct, but vou abrir sounds more natural in normal conversation.

Yes. Portuguese often drops subject pronouns when the verb already makes the subject clear.

So these both work:

Assim que a entrega chegar, eu vou abrir o pacote na cozinha.
Assim que a entrega chegar, vou abrir o pacote na cozinha.

Including eu can add emphasis, contrast, or just reflect the speaker’s style.

Because the sentence starts with a time clause: Assim que a entrega chegar. In Portuguese, when this kind of subordinate clause comes first, it is normally followed by a comma.

If you put that clause at the end, the comma is usually not needed:

Eu vou abrir o pacote na cozinha assim que a entrega chegar.

They are related, but not the same.

a entrega refers to the delivery or the arrival of what was ordered
o pacote refers to the physical package

So the sentence first talks about the delivery arriving, and then about opening the package itself.

Because em + a contracts to na in Portuguese.

So:

em + a cozinha = na cozinha

This kind of contraction is standard and expected.

Other examples:

em + o = no
de + a = da
de + o = do

Portuguese uses definite articles more often than English does.

Here, a entrega and o pacote refer to specific things that are already understood in context: the expected delivery and the package that comes with it.

Leaving out the articles would usually sound less natural here.

Yes. Quando a entrega chegar is grammatical and natural.

The difference is mostly one of nuance:

assim que = as soon as, with a stronger sense of immediacy
quando = when, more neutral

So assim que emphasizes that the package will be opened right after the delivery arrives.

Yes. A very natural alternative is:

Eu vou abrir o pacote na cozinha assim que a entrega chegar.

The meaning stays basically the same. The difference is mostly focus:

Assim que a entrega chegar... puts the time condition first
Eu vou abrir o pacote... puts the main action first

Both are common.

It most naturally tells you where the action of opening will happen.

So the idea is:

I’m going to open the package in the kitchen.

In other words, na cozinha modifies abrir, not pacote.

If you wanted to make the package itself sound like it is located in the kitchen, you would usually need clearer wording or more context.

It looks like an infinitive, but in this sentence it is functioning as the future subjunctive.

That is one of the tricky things about Portuguese: with many regular verbs, some verb forms are identical in spelling.

You can tell it is not a true infinitive here because:

assim que is followed by a conjugated verb for a future event
the subject is a entrega
the clause means when the delivery arrives, not to arrive

So in grammar terms, it is future subjunctive, even though it looks like the infinitive form.

A more formal version would be:

Assim que a entrega chegar, abrirei o pacote na cozinha.

That uses the simple future abrirei instead of vou abrir.

It is correct, but in everyday Brazilian Portuguese, vou abrir is usually more common and more natural.