code: public Completable addProduct(Product product) {
      List<Product> updatedShoppingCart = new ArrayList<>();
      updatedShoppingCart.addAll(itemsInShoppingCart.getValue());
      updatedShoppingCart.add(product);
      itemsInShoppingCart.onNext(updatedShoppingCart);
      return Completable.complete();
    }
docs: Adds a product to the shopping cart