No, it is not ribbis. You are not allowed to charge extra for late payment, but you are allowed to give a discount for early payment. If payment is due now, and for a higher price you allow the customer to pay at the end of the month, that is ribbis. But if payment is not due till the end of the month, you are allowed to offer a lower price if the customer pays now. The same applies to gift certificates: payment is not due until the goods are purchased, so it’s OK to offer a discount in return for payment in advance.