salesorderID = salesorder.get("salesorder_id");
organizationID = organization.get("organization_id");
sumOfLineItem = 0;
// Loop through line items and add sum to total
allLineItems = salesorder.getJSON("line_items");
for each  lineItem in allLineItems
{
    lineItemQty = lineItem.get("quantity");
    sumOfLineItem = sumOfLineItem + lineItemQty;
}
info sumOfLineItem;

// Create List for Custom Fields update
CustomFields = List();
CustomField = Map();
CustomField.put("label","Sum of Line Items");
CustomField.put("value",sumOfLineItem);
CustomFields.add(CustomField);
soUpdate = Map();
soUpdate.put("custom_fields",CustomFields);
response = zoho.inventory.updateRecord("SalesOrders",organizationID,salesorderID,soUpdate);
info response;