For accounting reasons, on your invoice you will see mentioned, "On Credit". This is because if we follow the order process step by step, the invoice is created right before the final payment. When you pay for your order, you receive an additional payment receipt, depending on the payment method you have selected (receipt from the bank, Paypal confirmation email etc.)

Those 2 documents, the invoice sent by Papaki.gr and the payment receipt are all the documents you need according to the law, to prove your transaction with Papaki.gr. What you need to do, is to simply forward the invoice to your accounting deparment.