Ebay have stated that they are gradually making Simple Delivery mandatory for all private sellers so you may not be able to opt out unless your item is under 100g and under £10 or an item which is too large or too expensive to use SD,
"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)
