Error when bill is zero (discounted)

i use Commerce, Commerce Node Checkout and Commerce PayPal (WPS) on my site and in combination with a 20€ discount (Commerce Discount + Commerce Coupon) it happens sometimes that a customer spends lesser than 20 € and the bill is zero, but he still needs to do the procedure to pay. so he get forwarded to paypal and then an error appears.
is it possible to verify if the amount is 0 and if so, you don´t get forwarded and the status changes to paid?