Learn Portuguese (Portugal): Nós vamos aos correios amanhã.

aos is a mandatory combination (contraction) of the preposition a (to) and the masculine plural article os (the). When you go to a place that uses a masculine plural noun, these two little words always merge into aos.

Why is the plural word correios used when talking about a single post office?

In Portuguese, the post office is referred to in the plural as os correios (literally 'the mails'). Because it is a masculine plural noun, it requires the plural article os, which is why we combine it with a to get the contraction aos.

The word amanhã means tomorrow, but vamos is present tense. Is that allowed?

Yes! Just like in English when we say 'we are going tomorrow', Portuguese frequently uses the present tense of the verb ir (to go) to talk about a fixed plan in the near future. The time word amanhã makes it clear that this happens later.

Can I drop the word Nós and just say Vamos aos correios amanhã?

Absolutely. In Portuguese, the verb form vamos already tells us the subject is 'we'. Pronouns like Nós are often left out unless you want to add emphasis.