In React/Redux, how to calculate a Total price for a shopping cart

I've searched for solutions on Google and SO but still cannot find an answer. They all stop at "Add item to cart" or "increase/decrease quantity" but never on calculating the Total, which is annoying!

In my app, there's a list of items where the user can enter the quantity if an item, which updates the its price. All I want to know is how do you sum up all the prices of all items in a cart into a Total price, with Redux and display it in my React app?

Also, if you can point me to any good shopping cart tutorial that actually goes beyond listing products in a cart, I'll be glad.

Action:

/**
 * @description UPDATE SINGLE ITEM PRICE WHEN USER ENTER QUANTITY
 *
 * @param{number} price
 * @param{number} quantity - enter by user from input
 */
export const updateItemPrice = (price, quantity) => dispatch => {
  const result = price * quantity;

  return dispatch({
    type: UPDATE_ITEM_PRICE,
    subtotal: result
  });
};

Reducer:

const INITIAL_STATE = {
  total: 0,
  subtotal: 0
};

const productsReducer = (state = INITIAL_STATE, action) => {
  switch (action.type) {
    // Update single item price
    case Types.UPDATE_ITEM_PRICE:
      return {
        ...state,
        subtotal: action.subtotal,
        total: // --> Here is where I'm stuck!!
      };

    default:
      return state;
  }
};