Quick question - VAT in Greece

Hi guys, we have a registered business in Greece, an online bookstore, and until now we offer only local pick-up, we are not even shipping yet. People come to collect their order in the bookstore, so no deliveries for Greece or Europe yet. Are we suppose to tick the box “Collect VAT”? I am not sure I understand the meaning of collecting VAT. Is it maybe an option only for those dispatching the goods? In case we should accept the Collect VAT option how/where do we enter our local (24%) vat ? e.g. for stationary, apparel etc the vat is 24% but for books or ebooks is 6%.

Thanks in advance!