It probably does approach 0. The truck is going anyway, most of the cost of a truck is wind resistance so adding more books up to the weight limit is close to zero.
Amazon is probably never shipping a single book, they are using analytics to decide if they should ship a case of books. If they think you are 90% likely to buy a book tomorrow but you are the only one who will it isn't worth the risk. If they decide 100 people are 50% likely to buy a book, sending a box of 100 books is a good investment - if 50 people buy tomorrow that implies 40 will buy the next day and 30 the day after and then 45 the next day and so on for a few weeks/months as sales slowly drop to zero.
Amazon is probably never shipping a single book, they are using analytics to decide if they should ship a case of books. If they think you are 90% likely to buy a book tomorrow but you are the only one who will it isn't worth the risk. If they decide 100 people are 50% likely to buy a book, sending a box of 100 books is a good investment - if 50 people buy tomorrow that implies 40 will buy the next day and 30 the day after and then 45 the next day and so on for a few weeks/months as sales slowly drop to zero.