The Stipend is your discount. If you want to buy a 2,000 CP worth of items this month the 400 CP Stipend is a 20% discount. If you only buy 800 CP worth of items this month the Stipend is a 50% discount. Even if you buy 4,000 CP worth of stuff in a month the Stipend still gives you a 10% discount. Everything you buy each month is discounted due to your Stipend.