Yes you can just use Amazon by Codisto to import Amazon orders into Shopify if you wish.  Although most merchants also use the solution for Offer/Listings creation and sync, some merchants just take advantage of the Order sync functionality.

You can choose which orders are automatically transferred to Shopify:

• All Amazon orders
• Managed Product orders (products linked/being synced)
• No automatic transfer of any orders (you can then choose which order to transfer by selecting)

And if these settings are for:

• FBA only
• FBM only
• Both

FBM orders decrement inventory in Shopify.  Once the shipping status and tracking data is updated on the order in Shopify, we automatically update Amazon with this data.

FBA orders do not decrement inventory in Shopify (as these use inventory from the FBA warehouse).  Merchants transfer FBA orders to take advantage of reporting/analytics/accounting integrations connected to Shopify.