Discussing this with eBay will not help. Simple Delivery is now mandatory for Private sellers, and currently only 2 postal services are available, Royal Mail, and Evri.
You could use your own courier, but your payout would be held for a minimum of 14 days before release, but probably even longer, as you are also a new seller and not using eBay's Simple Delivery.
I understand you have good results with your current courier, but is there any reason not to use RM or Evri ? The transaction would be far easier to use SD lables, especially as said, that you are a new seller.
@woo-2886