So lets say you want to buy a costume that costs 3000 pearls - and you have a discount coupon for 20% discount coupon.
So the actual price with the coupon is 2400 pearls - so you are like YAY I have 2500 pearls I can buy this now!
You have to have 3000 pearls (full price) before the coupon will be eligible to be used - so yes it will cost you 2400 pearls - but you have to have 3000 pearls on you.
So in other words - you are still going to be spending $30 to buy 3000 pearls even if you want to spend 2400 with the coupon.
They really need to make this point clear - as it's terribly misleading to players.