code: function ShoppingCart (objGoogleCheckout) {
  	var self = this;
  	
  	//todo: check the constructor name
  	assert.ok(objGoogleCheckout, "A GoogleCheckout object is the only required argument");
  	
  	self.gc = objGoogleCheckout;
  	self.settings = self.gc.settings;
  	
  	self.items = [];
  	self.shipMethods = [];
  }
docs: /*
  The `ShoppingCart` module provides methods for adding items, adding shipping options
  and submitting a shopping cart to Google. After submitting the shopping cart to
  Google, you will receive a URL to redirect your user to.
  
  There is no point in calling the constructor manually. You can get an
  instance of `ShoppingCart` like this
  
  ```javascript
  var gc = new GoogleCheckout(settings);
  
  gc.createCart(function (err, cart) {
  
  });
  ```
  
  Here is a complete example of building a cart and pushing it to Google
  
  ```javascript
  var gc = new GoogleCheckout(settings);
  
  gc.createCart(function (err, cart) {
  cart.addItem({
  name : 'Crayons',
  description : '42 Assorted Crayons',
  unitPrice : 3.99,
  currency : 'USD',
  quantity : 1,
  item_id : 'CRAY42'
  });
  
  cart.addShipMethod({
  name : 'FedEx Ground',
  price : 3.00,
  currency : 'USD'
  });
  
  cart.postCart(function (err, redirectUrl) {
  if (err) {
  //deal with the error
  return;
  }
  
  console.log(redirectUrl);
  });
  });
  ```
  
  @param {Object} GoogleCheckout A GoogleCheckout instance object.
  @return {Object} ShoppingCart Object
  @api public