The way I would understand it for your return if you don’t do ‘Check-N-Go’ and just arrive at the airport you will have to pay the charge for checking-in a bag (€12) and the new ‘airport check-in’ fee (€3).
However if you do ‘Check-N-Go’ then arrive at the airport and want to check-in your bag in then you will have to pay the bag charge (€12) and the boarding card re-issue fee (€3).
So either way you’re looking at the same price.