No matter what coupon I create, free shipping or xx% discount, when I set the conditions to be subtotal "greater than" or "equals or greater than", say, $100, the coupons always fail.
That means no matter what the subtotal is, smaller or greater than $100, the coupon always fail.
However, when I set the conditions to be subtotal "less than" or "equals or less than", the coupons always succeed.
This is really really weird. What could be wrong here?
Maybe one of the extensions override existing Magento coupon calculation with wrong logic? Which PHP file should I look to debug this? Function name to search for?