Simple Delivery is being rolled out for all Private sellers.
Some are under this already, that is, postage has to be purchased through eBay.
The message you received from eBay, I believe it's the 20th May, it's just an adjustment of some of the postage costs and sizes of parcels, as sellers already under SD were not finding those exact measurements / prices for their items.
This explains how Simple Delivery works:
https://www.ebay.co.uk/sellercentre/postage/simple-delivery?mkevt=1&mkpid=2&emsid=0&mkcid=8&bu=43952...
@talbotroad
