When a buyer is undercharged, the seller forwards

When a buyer is undercharged, the seller forwards a "debit note". A debit note is a document used to inform a customer that they owe more money on a purchase than was originally charged. In other words, it is a statement indicating that additional money is due because the original invoice was incorrect, either because of a mistake or because of a change in the original agreement. So, when a buyer is undercharged, the seller sends them a debit note to request payment of the additional amount owed.