Learn Hardcore Serbian: U pošti je veliki red, ali moja sestra kaže da će paket stići brzo. - There is a big line at the post office, but my sister says that the package will arrive quickly.

Breakdown of U pošti je veliki red, ali moja sestra kaže da će paket stići brzo.

Why is it u pošti and not u pošta?

Because the preposition u means in / at here, and when u shows location, the noun normally goes into the locative case.

So:

u poštu would usually mean into the post office (movement toward a place, accusative)
u pošti means in/at the post office (location, locative)

This is a very common Serbian pattern:

What does red mean here?

Here red means a line / queue, not order in the abstract sense.

So veliki red means a big line or a long queue.

The word red can have several meanings depending on context, for example:

In a sentence about the post office, red is naturally understood as a queue.

Why is it je veliki red and not something like ?