Personally, for the sake of the "realism" replies that the mods tend to give as an excuse to not implement the 1-plunder-limit, I think the reward for the convoy should be increased every time you beat off a plunder. 100% of the base amount sounds reasonable, since in that case a plunder attempt will end up generating a net of 35 coupon if successful( +50 plunderer -15 convoyer) or 30 if failed(convoyer) so it should be fairly reasonable. Note I didn't specifically state +30 coupon because there are fights over S convoys so might as well cover that by saying "base amount"
It is also perfect realistic for the guard to be paid extra when the mission turn out to be more hairy than expected.