You've tried to recreate this itinerary on Amex Travel and had no luck? Assuming you're also aware of the Amex Platinum IAP discount and that it doesn't apply here, I'd personally avoid booking this expensive a trip with 'FlyUs' - whoever they are.

There are plenty of disadvantages ranging from lack of access to an informed agent in the event of a schedule change, processing/refund fees that the airlines themselves would not charge, and OTA opacity around specific fare rules.

I use Amex Travel if the IAP savings are worth it or if there's no other way to directly ticket a mixed-carrier booking with an airline, and would use Expedia and such in exceptional cases, but almost never a no-name OTA on an international J ticket.