www.mountainwarehouse.com / seller took wrong sum for the order

My husband bought outdoor equipment from the company www.mountainwarehouse.com. He provided the card info and the payment was taken within 2 days after he has placed the order. But the seller turned to be really dishonest guy, because he took wrong sum from my husband. And he hasn’t warned him. My husband waited for the order, but accidently noticed the difference between the sums. After that the seller stopped to communicate with my husband. Please, help us to solve this problem and get money back.