First payment due - may be the same as the date of the loan for rents. See above "For the start date of the loan and first date of payment."
Extra Payments Amount - do you want to make one additional payment or a number of payments? Enter the quantity.
Extra Payments Start – enter the date on which you wish to start additional payments. The date does not align with the due dates of payment. You will want to make additional payments on the 15th to comply with your payment periods, because you are paying a loan monthly, and payments due on the first one.