Simple Delivery is mandatory now for all Private sellers.
You can of course use your own postage if you wish, but your payouts may then be held for a minimum of 14 days before release, and you have to remember if you had Seller pays for postage ( The old ' Free Postage), to then cancel that label for your refund for the unused label , and to do so within 14 days.
I'm not the greatest fan of SD, please don't get me wrong there, but it does have some benefits, in that if your item is lost in the post or arrives damaged in transit, eBay take over the refund, and you get to keep the payment for the item.
@paulo7435