The Bikini Bottom Post Office appears in the episodes, "Hocus Pocus" and "Dear Vikings." It also appears in the online game Postal Panic. SpongeBob writes a letter to the Vikings, but the Mailfish puts it in the bin. But a Viking, that works as a garbageman in Bikini Bottom, finds the letter and brings it to the King of all Vikings. Apparently it has a variety of P.O boxes, and mail trucks that have one notable driver who is usually scared by SpongeBob when he hides in the mailbox.
The post office is shaped like a cardboard box with all of the letters overflowing.
It is unknown if the stamp store is inside or connected in some way to the Bikini Bottom Post Office.