Where it is not practicable for the mailer to affix the exact postage to each piece or to affix the lowest postage price to all pieces in the mailing, the mailer may compute postage for the mailing as if the lowest price affixed to any piece in the mailing were affixed to all pieces. Additional postage is computed based on the difference between the lowest price affixed to any piece in the mailing and the price for each price level in the mailing. This computation must be documented to meet the basic standards in 3.0. No refund is paid for any piece where postage is affixed at a price higher than the lowest price claimed for or affixed to any piece. The total additional postage must be paid either by advance deposit account or by a meter stamp affixed to the back of the accompanying postage statement.