They have a volume agreement with the postal services. So for example every month they pay 100$ for shipping 100 items. However if some month they have only shi

They have a volume agreement with the postal services. So for example every month they pay 100$ for shipping 100 items. However if some month they have only shipped 90 items they still have room to include another 10 items no matter what they sale it for. The real story is that they are giving things for free and no one can beat them. How do I know this? Just through some rational thinking.