For pool milk received during each month from dairy farmers or cooperative associations of producers, each handler shall pay per hundredweight not less than the prices set forth in this section, subject to the differentials and adjustments in sections 20.51 and 20.81 of this Part. Any handler who purchases or receives milk during any month from a cooperative association of producers but does not operate the plant or the unit receiving this milk from producers shall pay the cooperative association on or before two days before the last day of the month if paid by check, or the last day of the month if paid in cash or cash equivalent, at not less than the lowest class price pursuant to this section for the preceding month for milk received from such cooperative during the first 15 days of the month, and shall pay the cooperative association on or before the 15th day of the following month the balance due for milk received during the month from such cooperative at not less than the class prices pursuant to this section subject to the differentials and adjustments set forth in sections 20.52 and 20.81 of this Part applicable at the plant at which the milk is first received from the cooperative association. Such payments to a cooperative association shall be deemed not to have been made until the payments have been received by the cooperative association.