The buyer pays Ebay for the label - that money is not paid into your account.
If you choose not to use Simple Delivery and the Ebay label, the buyer will be refunded the cost of the label by Ebay when the label is not used.
Business sellers do not have to use Simple Delivery.
You describe yourself as a 'stockist' and the items you sell as 'stock'.
If you upgrade and trade correctly from an Ebay business account, you will not have to use Simple Delivery.
"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)