The Invoice Date is found on an individual Invoice entry. It represents the date the invoice was issued, which in typical accounting terms is the ship date of the product ordered from the manufacturer to the customer.
The Payment Date is found in a Payment Entry. It represents the date the commissions were paid to the sales agency, which is typically noted as the check date on the commission check from the manufacturer. For accounting purposes at the sales agency, the Payment Date may also be entered as the deposit date of the check to account for revenue recognition practices of the agency.