In order to get your purchase tax refunded, you must, whenever you buy anything, fill out a (Tax -Free) application which is available at any shop.
keep these forms available with you at the airport when you are leaving Greece to any country outside the Schengen area, you must go to the customs office to register your purchased items along with the tax -free application, the customs office will stamp this application after reviewing the data stated thereon , then you must go to the Office of (EuroChange) to get the refund of the amount of tax you paid.