I guess I will stick my oar in: I have been selling used books on
Amazon for five or six years.
There is a clear difference between buying 'from Amazon' and buying from a third party (like me) 'through Amazon'.
For instance: Sue Grafton's lasted book "U is for Undertow". Amazon itself stocks and sells the book (new). I can pay $3.99 for overnight shipping, or opt for free shipping which takes an extra day or two (I am a member of Amazon, paying the $75.00 annual fee).
Now, you have third party sellers that are selling "new", "used" or "collectible" books. If you buy from them you will pay $3.99 for 'standard' shipping. Some of the sellers may offer quicker shipping for another few dollars.
If you buy from a third party you are not dealing with Amazon. If you buy from me, for instance, after you check out Amazon sends me an email notifying me of the sale. I then go into my account, click on my 'orders' button, and then I click on your shipping address and print the shipping label. I then pack up the book and mail (using media mail, in most cases). I will then email the buyer to let them know I received the order and to tell them when I will mail and when they may reasonably expect to receive the book.
After I mail the book I go back on Amazon and 'confirm shipment'. Only at that time will Amazon credit me the money (the price of the book, plus $3.99 for shipping, less Amazon's fee).
I wanted to detail this because sometimes a person will order a book from me thinking I am Amazon, and then email me about other items they have purchased, which, of course, I have no idea about.