Is it possible to create a coupon voucher for lets say $ 1000 if a person then makes a purchase of $100 they are left with a coupon to the value $900 they can then make another purchase of $400 which leaves them with a coupon that has $500 remaining on it
Coupons will only have fixed values for each use during the checkout workflow, so it will probably require code customization to create that kind of option.
