You have the alternative of paying for university fees today for a payment of $15,000 or, you can select a payment plan where you pay $8,000 in 6 months from today and another $12,000 in exactly 18 months from today. If the interest rate is 9.9%p.a. compounding monthly, what is the advantage that the payment plan has over the upfront payment?

(expressed in present day value rounded to the nearest cent; do not show $ sign or comma separators; if the payment plan is more costly than $15,000 today, your answer will show a negative eg. -300.35)