If you plan to include in your luggage the products listed in the invoice, then you should go to customs before security control with your invoice, passport, flight ticket and unused goods. Note that if you don't have your flight ticket, you will probably be asked to get the ticket at the check-in desks, then go to customs and then drop-off your luggage at the check-in desks again. 

If you plan to bring the products onboard, then it does not matter whether you go to customs before or after security control (warning: make sure there is a customs office after security control if this is your plan). In any case, you should show customs your invoice, passport, flight ticket and unused goods. If you decide to visit customs before security, note that if you don't have your flight ticket, you will probably be asked to get the ticket at the check-in desks and then go to customs.