The actual system would work perfectly fine if they swap the ingots cost between the features, in the sense that you can claim all the ss convoys you get for free and to claim a 2nd or 3rd plunder attempt you need to spend 20 ingots. This way every free to play would simply run convoys knowing they can potentially get 140 coupons for free out of them while only cashers whould think to plunder, knowing that those 150 coupons they got still costed 40 ingots.