Hello again

The single/multiple shipping is referred to the amount of items for each product added to cart, so for example adding 5 items of product X to the cart with shipping cost of 5USD the shipping can be 'single' and be for example 5 USD for all 5 products or 'multiple' and calculated equal to 25 USD

Having a global shipping cost for all the cart that contains different products is normally not possible with default features.

The email that is sent when users add to cart a product to place order is optional can be disabled if needed. This is done in this phase because after that all is processed on Paypal servers and you can't get notified anymore by Joomla site about the new order.