Yes, your parents can pay less than the amount of the charge but they can not pay more.
To Pay Less than the Charge Amount:
In the Parent Portal go to the Make Payment Screen.
Select the charge or charges to be paid by checking the box to the right.
Click in the Payment Amount Field and enter the amount that you'd like to pay.
Enter the Credit Card information if not already saved.
Click Pay Charge.
Click here for complete instructions.