Any company selling products on Amazon has a choice to ship the product that they sell on Amazon platform themself or pay a monthly fee where Amazon ship the products to the buyers with free shipment and quick turnout.
The Individual FBA plan costs $0.99 per unit sold, and the Professional plan costs $39.99 per month no matter how many units you sell.
In able for Amazon to ship your products, you need to send your product to Amazon facilities around the country. So if you buying stuff from China to sell on Amazon, then instead of shipping it from China to you and you ship it to Amazon, you can ship it directly to Amazon facilities. That is all.