Maybe something (contrived) like this providing no-op defaults?

  total = calculateOrderTotal(user.order);
  if (user.isPremiumMember) {
    total = total * 0.9;        // 10% discount

  total = calculateOrderTotal(user.order);
  discount = calculateDiscount(user);  // Returns 0.9 or 1.0
  total = total * discount;