The following code contains the backend function for updating quantity in the Cart_WithService .ssp file:

function service(request,response)
   // get incoming http parameter
   var quantity = request.getParameter('quantity');
   nlapiLogExecution('DEBUG', 'quantity', quantity);
   // get active order in web container   
   var order = nlapiGetWebContainer().getShoppingSession().getOrder();   
   // get the items out of the order
   var items = order.items;
   var orderItemId = items[0].orderitemid;
   nlapiLogExecution('DEBUG', 'order item id - line 1', orderItemId);
   var currentQuantityLine1 = items[0].quantity;
   nlapiLogExecution('DEBUG', 'order curr qty - line 1', currentQuantityLine1);
   var updatedItem = new Object();
   updatedItem.orderitemid = orderItemId;
   updatedItem.quantity = quantity;
   var itemsJSON = JSON.stringify(order.getItems());
   // send items back out as JSON,