after discount_copy(day[0], 10): Rp 24500 (unchanged)
after discount(&day[0], 10):     Rp 22050
orders of the day, total Rp 104550:
  #1 Andi Pratama    3.5 kg  Rp  22050
  #2 Budi Santoso    2.0 kg  Rp  20000
  #3 Citra Lestari   4.0 kg  Rp  24000
  #4 Dewi Anggraini  5.5 kg  Rp  38500
largest: #4 Dewi Anggraini
with delivery fee: Rp 43500 (day[3] changed)
sorted by amount, largest first:
  #4 Dewi Anggraini  5.5 kg  Rp  43500
  #3 Citra Lestari   4.0 kg  Rp  24000
  #1 Andi Pratama    3.5 kg  Rp  22050
  #2 Budi Santoso    2.0 kg  Rp  20000
