。。
The post office is along this street. You'll spot it easily.
Literal
Post-office [topic-は] street-along-[at-に] exist-[polite-ます]. Immediately understand-[polite-ますよ].
通り沿いに ('along the street') uses the suffix 沿い ('along, alongside') — built from the verb 沿う ('to run along, follow the line of'). わかりますよ here means 'you'll be able to find/recognize it,' extending わかる's basic sense of 'understand' to 'identify, recognize.' The friendly よ softens the reassurance.