function getRefund(uint _courseId) public {
    require(customers[msg.sender][_courseId].userId > 0);
    require(customers[msg.sender][_courseId].refunded == false);
    require(customers[msg.sender][_courseId].purchasedAt + (3 hours) > now);
    customers[msg.sender][_courseId].refunded = true;
    msg.sender.transfer(customers[msg.sender][_courseId].amount);
    RefundPayment(_courseId, customers[msg.sender][_courseId].userId, msg.sender);
  }