Answer: Purchase stage
Explanation: This is the second last stage in the buying process. Until this stage the customer must have decided to make the purchase. However it is not guaranteed that the customer will do it as any small issue could lead to change in mind.
In the given case, Sandra is giving additional services as the customers on her sight decides to purchase the book but cancels the order after.
Thus, from the above we can conclude that the given case relates to the purchase stage.