Just on your left as you leave the main central exit of Kyoto Station is Kyoto's Central Post Office.
Japan's Central Post Offices are invariably located near to the main railway station and provide a 24-hour window for posting mail and using the ATMs which accept foreign credit cards.
Kyoto Central Post Office is close to the Rihga Royal Hotel and the APA Hotel Ekimae Kyoto.
Kyoto Central Post Office
Higashi Shiokoji-cho 843-12
Rough Guide To Japan