One should try to the maximum extent to get something in writing from the customer, If it is a written communication, then there are two ways to deal with such a situation, If the customer refuses to pay the outstanding amount this event would be either verbal or written communication and If it is a verbal communication, then care should be taken to send a few reminder letters in a polite manner indicating the amount due from the customer.
Here`s my computation:
outstanding balance = $21,772.00 interest = 6.8 %
$21,772 x .068 = $1480.496$21,772 + $1480.496 = $23,252.496
$23,252.496 / 48 = $484.427
meaning you have to pay $484.427 monthly for 4 years