> ...most suppliers do FBA which tacks on a more expensive shipping charge than you would have otherwise with USPS.
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.