Post offices are available in cities and bigger rural towns as well. However postal services are not very reliable, especially for foreign shipments, so we recommend to use them only in important cases.
On the other hand, you do not necessarily need to go to a post office if you want to send a postcard to your loved ones in your home country. In most cases you can also leave them at the reception of your hotel, the staff will happily take it to the nearest post office for you.