def increment(self, quantity=None):
        data = {}
        if quantity:
            data['quantity'] = self._normalize_quantity(quantity)
        
        response = self.subscription.customer.product.client.make_request(
            path = 'customers/add-item-quantity',
            params = {
                'code': self.subscription.customer.code,
                'itemCode': self.code,
            },
            data = data,
            method = 'POST',
        )
        
        return self.subscription.customer.load_data_from_xml(response.content)
Increment the item's quantity by the passed in amount.  If nothing is
dataset/ETHPy150Open SeanOC/sharpy/sharpy/product.py/Item.increment