$60.00 order ...

20% off Discount Coupon ...

Sub-Total: $60.00
Flat Rate (Best Way): $4.95
Discount Coupon: XXXXX : -$12.00
Total: $52.95

10% of $60.00 is $6.00
20% of $60.00 is $12.00

What isn't computing correctly?

Perhaps a new battery for your calculator?