Post office usually in many countries is a Government institution that operates as a mailing service. USPS is the biggest in the world with third biggest employer in the US.
Mailing service is delivered using a postal code and address along with Post office box numbers. Every country has a postal code, then every city has a postal code and then the area has postal code with addresses and every post box has a post box numbers. Using these postal numbers mails is delivered to the correct address.
Every post office has an opening and closing time for every week. Kindly look at the office times here: https://postofficenearme.xyz
Post office near me website contains list of all the post offices worldwide, their numbers, and office hours.