Amazon then gives me a pre-paid shipping label.
I find the whole process enjoyable. "How much will Amazon give me for this?" is an exciting game. Then things go in the box, and you can visibly see the consumer weight you're lifting off your shoulders. Then you get to see the fruits of your labors with a newly empty bookshelf.
The money they give me for it is only a small part of the benefits I feel from doing it. I try to do the same with Goodwill donations of old clothes, but I have to admit some laziness in actually getting down to the Goodwill (which is essentially zero difference from driving the box to the UPS Store. Procrastination is a weird and wonderful thing.)