Fulfilment by Amazon (FBA) is a service from Amazon that takes care over everything: shipping, returns, refunds, product warehousing, picking, packing. Fulfilment by Amazon may not be the best option if you sell many low-value items or large dimension products.
If you want to join the Amazon FBA program, you need to pay storage fees and fulfillment fees. The more inventory you have, the more you need to pay. And if your inventory is stocked for longer than 180 days, costs will also go up.