It depends on what you want to do exactly.
If you want to sync inventory and automatically fulfill Shopify using your Amazon FBA inventory, you can use Order Automator to connect that to any Amazon marketplace in the world, route multiple marketplaces, plus map things like shipping options and skus, customize your packing slip notes, etc.
If you want to import Amazon FBM orders to your Shopify store, I used an app called Codisto a few years ago to do that and it worked well for us.
When I sold on Amazon I specifically tailored the Shopify listing and the Amazon listing differently, because the customer and requirements are different, so I didn't want to use an app, but if you wanted to sync those, I would use csv to export / import, or search something like "shopify app to sync amazon products". I would assume any app that advertises they sync products, that has good reviews would do the job well.
In summary, decide what you want to do exactly then search for that functionality in the Shopify app store. Contact the apps you think may be a good fit, and see how they respond. If you get good vibes, the reviews are good, and the solution fits, give it a go.
