Amazon.com has free shipping for purchases equal to or more than $25. Amazon.com encourages their customers to buy a second book. One reason the customer does is to not pay for shipping twice.
The price of shipping is allocated to all products. If the customer buys less than $25 they pay shipping twice: the price allocated to the price of the product plus the amount charged for shipping.
The only way the customer can avoid paying twice is to take advantage of the offer for free shipping which isn't really free and should be "no additional charge."
