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...and you can do this after you do a search and see results and get to where you have additional filtering options. I'm guilty of using "Show only Prime offers" or whatever that one is called. After becoming a seller that does FBA and seller-fulfilled, I use that filter less. There are great small businesses that still ship very fast, and at competitive prices and since I'm one now, I feel their pain and want to support them too. I can also be patient on a lot of my shipments and don't require 2-day shipping.
If your inventory sits too long in their warehouse, the fees go from "regular" storage fees to "long-term" storage fees. Those fees are much higher, which incentivizes the seller to put the stuff on sale to blow it out of their warehouses, or you can have the inventory sent back to you.
Also, fees vary by category and season. Around the holidays, you'll see them making a push to increase long-term storage fees to make way for the stuff that will actually sell, and on categories which are hot for the holidays, you might see them providing incentives with lower storage fees.
I'm not the primary person that deals with Amazon FBA, but he sits next to me. Don't take what I'm saying as the gospel, but this is based on somewhat second hand experiencing and reading some of the emails that come through. My partner pays more attention to those details.
This is not universally true. As a seller on Amazon, using both FBA and seller-fulfillment, there is a lot of "it depends" in all of this.
Some items we can ship cheaper, especially to local zones. But as we're on one coast and we have a lot of customers on the other coast, FBA can be a real saver on the shipping charges.
Size and weight of the package affect the shipping charge, but on some products we sell, we find the fulfillment fees to sometimes get as low as less than 1/2 of what it would cost us to ship direct, even by the cheapest method we can use.
If you're looking at the fulfillment fees compared to what you get charged to ship a package, make sure you're also considering that Amazon helps distribute the products to regional fulfillment centers. It's Amazon's employees that pick and pack the orders. Amazon pays for the box it ships in. Amazon handles customer service and return processing, or reshipment if the item is damaged in transit, amongst other things.
Admittedly, this is not easy to consider all of the pros and cons, expenses and savings. FBA can also be a difficult beast to deal with when they don't check in your inventory, temporarily lose 300 units. Having to deal with Seller Support will cost you in terms of stress and its effects on your receding hairline, or your liver if you need to keep a bottle on your desk as you try to get problems resolved.