code: func (item OrderItem) Amount() float32 {
  	amount := item.SellingPrice() * float32(item.Quantity)
  	if item.DiscountRate > 0 && item.DiscountRate <= 100 {
  		amount = amount * float32(100-item.DiscountRate) / 100
  	}
  	return amount
  }
docs: // Amount order item's amount