Hi Basara,
We are making an extension for payment via invoice after admin adds actual shipping charges - having issue with hook into PayPal Commerce to let me change message on the fast checkout success page.
Sending you a PM with details.
Thanks.
Probably you should to hook controller public_html/extensions/fast_checkout/storefront/controller/responses/checkout/pay.php method _process_success
You can write _updateData hook and use $that->session->data['processed_order_id'] for order recognizing