You can look up the costs - and even sign up for the seller programs - really easily. For small items, it looks like FBA (Fulfillment By Amazon) costs $2.50 a p

You can look up the costs - and even sign up for the seller programs - really easily. For small items, it looks like FBA (Fulfillment By Amazon) costs $2.50 a pop to handle prime shipping/customer service/returns/etc. And I think you have to pay for shipping to their warehouses, too. Plus warehouse space, but that looks very cheap for small volumes at a glance. So if you see something that costs $5 with multipack options of 3-4/$10...well, there you go. And you too can make any product available to a huge audience with 2-(ish)-day shipping...no wonder they're going like gangbusters.