Many tourists may visit the post offices in Japan during their stays. Many post offices in urban areas are located at the ground floor of the buildings which other offices and shops are located in. However, there are many post offices in non-urban areas which have their own buildings.
Then, the post offices located at the tourist areas have distinguished styles of their buildings as you can see in the picture.
By the way, current postage to send a post card to Europe from Japan is just JPY 70/card. Parcel by economy is JPY 1800/up to 1 kg or JPY 2500/up to 500 g by air. Please note that post card can be send to anywhere for JPY 70 but the postage of the parcel depends to the destination.
Post offices are mostly open from 09:00 to 17:00, except central post offices in prefectural capitals and large cities which can be open till later. If you want to buy a stamp, you can buy it anywhere at the convenience stores.