Coupons help to fill slots in the Merchant Ship.

To use a Coupon, instead of goods, do the following:

1) Choose an empty box on the Ship.

2) Tap a blue button with a Coupon icon to use it.

You can get Coupons as a reward for completing some quests in events and expeditions. It is also possible to purchase them in the Bank as a part of some packs.