El casillero postal está en la sucursal de correos.

Spanish uses estar for location, so está en la sucursal de correos means is located at the post office branch.

A quick rule of thumb:

• ser = identity, classification, inherent characteristics
• estar = location, condition, temporary states

So:

• El casillero postal es... would sound wrong here if you are talking about where it is.
• El casillero postal está en... is correct because you are giving its location.

Yes, casillero postal is a common way in some Latin American varieties of Spanish to say P.O. box.

However, this can vary by country. Other terms you may also see are:

• apartado postal
• casilla postal
• sometimes other local expressions

So casillero postal is perfectly understandable in many contexts, but learners should know that the exact term can change depending on the region.

Because sucursal de correos is a normal way to say post office branch or postal branch.

Here, de correos means something like of the postal service / of the mail.

Compare:

• casillero postal = postal box / P.O. box
• sucursal de correos = post office branch

Spanish often uses different structures where English might reuse the same adjective. So instead of forcing postal everywhere, Spanish naturally says de correos in many cases.

Yes. Sucursal means branch or branch office.

It is used for many kinds of organizations, for example:

• la sucursal del banco = the bank branch
• la sucursal de la farmacia = the pharmacy branch
• la sucursal de correos = the post office branch

So sucursal is not specific to the mail system. It is a general word for a branch location of a larger institution or business.

Because nouns in Spanish have grammatical gender.

• el casillero is masculine
• la sucursal is feminine

That is why the articles change:

• El casillero postal
• la sucursal de correos

This gender does not necessarily have anything to do with biological sex. It is just part of how Spanish nouns work, and you usually need to learn the gender along with the noun.

In sucursal de correos, the phrase de correos works like of the postal service / of mail and does not need an article.

This is very common in Spanish with de phrases:

• tienda de ropa = clothing store
• taza de café = cup of coffee
• oficina de turismo = tourist office
• sucursal de correos = post office branch

You could sometimes see a phrase with an article in other contexts, but here the version without an article is the normal one.

Sometimes in everyday speech, people may say something shorter depending on the country, but it is not always the same thing.

• el correo can mean the mail, the post office, or even email, depending on context.
• la sucursal de correos is more specific: it clearly means the post office branch.

So if you want to be clear and precise, en la sucursal de correos is a good choice.

Not everywhere. Vocabulary for P.O. box varies a lot by country.

You may hear:

• casillero postal
• apartado postal
• casilla postal

A native speaker from one country may prefer one term, while another country uses a different one more often. This is very normal in Latin American Spanish.

A broad Latin American pronunciation would be roughly:

el kah-see-YEH-roh poh-STAHL es-TAH en lah soo-kur-SAHL deh koh-RREH-ohs

A few useful notes:

• ll in casillero is usually pronounced like y in much of Latin America.
• rr in correos is a rolled or trilled r.
• The stress falls on:
  • casiLLEro
  • posTAL
  • esTÁ
  • sucurSAL
  • corREos

Yes, but the original order is the most neutral and natural:

• El casillero postal está en la sucursal de correos.

Spanish can sometimes move things around for emphasis, but not all changes sound equally natural.

For example:

• En la sucursal de correos está el casillero postal.

This is grammatical, but it sounds more marked or emphatic, as if you are emphasizing the location.

For a learner, the original sentence is the best standard version to use.

Usually no, not in this sentence.

• en = in / at
• está en la sucursal de correos = it is at the post office branch

That is the natural preposition for location here.

Other prepositions would change the meaning:

• junto a la sucursal = next to the branch
• frente a la sucursal = across from the branch
• dentro de la sucursal = inside the branch

So if the idea is simple location at the branch, en is the right choice.