Simple Delivery is now mandatory for Private sellers.
You can use your own postal service and price, but unless you can manually change the tracking number to that used, your payout will be held for a minimum of 14 days before release. If tracking cannot show delivery for any lost item, you will need to refund that buyer.
There are bonuses with SD, if the item is lost in the post, or the buyer received an item which was damaged in transit, the eBay pick up that cost, they refund the buyer, and you get to keep the payout for the item.
@tillyoui