One Stop Order Processing can not directly import FBA orders. The solution to import Amazon FBA Orders is shown below:
1 In the Import Orders dialog create a new Amazon Import
2 Set this to a suitable name and use a wildcard file name to pick up all TXT files (i.e. *.txt). For example, as shown below:
The folder to look in should be the download folder for your internet browser (where you connect to Seller Central). You can also make this a different folder (e.g. C:\Users\svertor\Downloads\Amazon\*.txt) so you make sure that you only process the required Amazon orders.
After confirming and closing the import (click OK) you should accept the settings (updating any relevant FBA requirements)
3 To make sure that you only process the new FBA order files tick the Rename import file after import option. You should also make sure that this is for this import only by ticking the Use different order import option for each import type (unless already ticked). Shown below:
4 Login to Amazon Seller Central
5 Select the FBA Reports Page, link below
https://sellercentral.amazon.co.uk/reportcentral/AFNShipmentReport/1
6 This will display the daily shipment reports