I sell my old textbooks often on Amazon. Perhaps the following will be helpful:
I don't usually sell a book if the listing price is less than about $7.
For packaging, I save all of the shipping sleeves and book sized boxes that books come in when I order them from Amazon (usually used- I never buy books on campus). Also, because I have the lower cost student Amazon prime which includes free shipping, I usually order my books one at a time to maximize the number of mailers I receive rather than big boxes. I pop the book I sold right back in them, peel off the label and relabel. The only expense is a roll of packing tape to secure the box and cover the shipping label.
For cheap books I always ship them media mail. Usually just about $3. If someone paid a lot of money for a book I might ship first class. But a heavy book is always more expensive and you have to price it accordingly.
Sometimes, I just sell the book back to Amazon for credit. It is worth less money but sometimes it is pretty close in dollar amount and it makes it easier and there is no shipping cost.
I have had an issue one or twice with a book getting there very slow, but honestly, if someone is paying less than half the going price, I don't think they mind waiting a bit. Holiday time shipping is always slower. And I nearly always ship the same day it is ordered. If I don't get it out by the second day, I will pay extra for first class or priority. And once I needed to credit someone because it just got too delayed and they got it elsewhere. I asked them to return the book which they did and then I gave them credit and resold it.