The WC_Cart->discount_total argument is deprecated since version 2.3. After tax coupons are no longer supported. For more information see: https://woocommerce.wordpress.com/2014/12/upcoming-coupon-changes-in-woocommerce-2-3/

Solution on line 131 class-pay4pay.php

Change
$calculation_base -= $cart->discount_total + $cart->discount_cart;
for
$calculation_base -= $cart->get_total_discount() + $cart->discount_cart;

Best regards.