Sending a postcard from Crete is easy. You only need a postcard and stamps. The service is provided by ELTA (Elliniki Tahydromia). Post offices (Tahydromia) can be identified by a yellow sign. Also the mailboxes are yellow. They have different dimensions. Sometimes it is hard to find a mailbox. Often you'll find them on or near the central square of a town and in the side streets of the main street of a village.
Logo of ELTA
Stamps are for sale at post offices, but often also in the shops where postcards are sold. According to various guidebooks postage costs more in that kind of places, but that is incorrect. To send a postcard to other EU countries you need a 65 cent stamp. Letters should officially be delivered within 4 days, but it often takes longer. There is also an express service (First Priority). This will cost approximately € 1.20 extra. The sending of the mail would be slightly faster. The mailboxes of the express service are red. But if you need to send express mail you'd go to the post office.
If you can not find a mailbox you can also hand the postcards over to the reception of the accommodation where you stay. You should not do this if you don't trust the staff of the accommodation
Packages can be sent from any post office.