After adding permissions, do not log back into the sub-account because it will require you to set up 2-factor authentication.
If 2-factor authentication is enabled, we cannot access the sub-account!
Moving forward, simply input your shipment contents to your Google spreadsheet before the shipment is delivered to us.
Be sure the product listings exist in your Seller Central account (primary) and set them to "Fulfilled by Amazon" (not "Fulfilled by Merchant").
We look forward to working with you!