You will have to use Simple Delivery. As the buyer pays Ebay directly and you never have the payment for postage, I would say the answer is 'no'. You can't buy a discounted service with money which you never receive.
You won't have the freedom to buy postage. Ebay will send you a label or QR code so you can print the label or take the QR code to the drop off point.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)