Update: Fourth conversation with eBay agent, he said the escalated complaint is still with the I.T dept. He said to list it outside eBay direct with buyer on this occasion as eBay are unable to list it correctly on Simple Delivery.

But I’ve now heard back from my buyer & its inconveniently late for her to receive it, so she has pulled out. A total waste of money time and energy thanks to eBay’s failed Simple Delivery. 
Incase it is helpful to anyone- my parcel is 106cm by 73.5cm by 6cm so large & flat. Weighs 6.1kg. Yet Simple Delivery only offers me ‘Medium’ parcel. If sent by evri it will cist £9.24 including insurance & proof of delivery.