Amazon FBA as a vendor is different than Amazon as a site.
Amazon as a site is when Amazon is connected to Spark Shipping as a selling channel and Spark Shipping is used to fulfill orders by pulling the orders from Amazon and sending them to your suppliers/vendors, Spark Shipping can also update the inventory in that channel, push listings and update prices.
Amazon FBA as a vendor is when you want to connect your FBA inventory to be used as an inventory feed. Spark Shipping can pull stock quantities from this FBA vendor and publish those quantities into your site. Spark Shipping can also send orders to FBA and pull tracking updates from there. Think of it as any other supplier/vendor/warehouse that you have.
To add FBA as a vendor to your site in Spark Shipping, go to Vendors > scroll down to find the list of built-in integrations and find Amazon FBA > Add Vendor.
Please note: Amazon FBA as a vendor does not pull costs/prices.