By default on Pango, buyers pay for shipping. The shipping fee buyers pay is used to create a prepaid USPS label that gets sent to the seller. For this reason, sellers do not have to pay for shipping on most Pango orders. All you have to do is print out your label and send out your books!
The exception to this is if you have a free shipping offer set up on your account (which is a totally optional feature). For more information on how free shipping offers work, check out this article.