Learn Turkish: Posta bugün geç geldi. - The mail came late today.

It is acting as an adverb because it describes the verb geldi (came). In Turkish, most adjectives can be used as adverbs without adding any special suffixes (like "-ly" in English). You just place the word right before the verb.

Why is bugün placed after the subject (posta) instead of at the beginning of the sentence?

Both are correct! Placing time words like bugün (today) at the start of the sentence is very common, especially if you want to emphasize the time. However, the standard, neutral word order in Turkish is Subject + Time + Adverb + Verb, which is exactly what we see here.

Is posta the same thing as the post office?

Not quite! Posta means the "mail" itself or the postal service in general. If you want to talk about the physical building, the word for "post office" is postane.