。
The post office is directly across from that shop.
Literal
Post-office [topic-は] that shop-of exactly directly-opposite [at-に] exist-[polite-ます].
真向かい ('directly opposite, straight across') is another 真ん-prefix spatial word. Common in directions when the speaker wants to emphasize that the thing is facing the reference point exactly. Pair with 真ん前 ('directly in front') and 真後ろ ('directly behind') for a locative triple.