Visual Form Builder Pro

Hi Support,
I have created a form for items to be purchased. Now we have coupons for customer to redeem , and no matter how I created another line with -$xx . The final sum do show the correct subtraction but the link to paypal does not work.
If I do not click the coupon -$xx , and the final sum remain as per the item listed. The link to the paypal work. Please advise .