The bonus icon will pop up after you've finished 20 orders in the Trade House. Click the icon and a random bonus will be added on your orders that have arrived.
Please note that:
1. The bonus will only take effect for one of the rewards (the one with an icon of "+X%" )
2. If you have declined one order before you click the bonus icon, and a new order has not arrived yet, then the order will be unable to receive the bonus after it's refreshed.