Can I have a product available on FBA and Merchant Fulfilled at the same time?
It's currently not possible to have a single WooCommerce product available to be fulfilled by Amazon (FBA) and by the merchant (you) at the same time. This would require two SKUs to be assigned to the same product - which isn't possible in WooCommerce.
WP-Lister for Amazon will enable FBA automatically for products which are currently in stock in FBA. When there is no stock left on FBA but still some stock remaining in WooCommerce, it will fall back to list the product as Merchant Fulfilled using the stock level from WooCommerce.
The only way to offer an Amazon product via FBA and Merchant Fulfilled at the same time would be to create two products in WooCommerce with different SKUs - and set the visibility option to "hidden" for one of them.