You don't. It's that simple. You get them for spending 50g in trade fair of wonders, then when you check "for regular customers" tab, you'll see some items are for sale for 10% off. At 5 coupons there'll be more items for 20% off and 10 coupons it's 30% off.
I am handing out equal judgment to everyone at an affordable rate, call me Fix Price, When the king dies, make sure his grave is King-Size.