What's common between radio commercials and dairy products? They both get a "return" after their positioning. According to an article by Costas Chardavellas at newsbomb.gr [4], it is a common practice in the Greek advertising market, for advertising agencies to require radio and television stations to return part of the profits from the commercials, at the end of the year. "The advertising companies profit from the economic decline of the media, by forcing them to return much more than 9.9% on the price of the spot, as the relevant law specifies [...] usually pushing for rates of around 26-28%." The relevant article even published a list of major clients and the refund rates which the advertising agencies require radio & television stations to pay, that go up to 31%!
Note also that this regular refund practice is a purely Greek invention and does not apply to any other country in the eurozone.