When you buy your own postage you have to add the tracking number and mark dispatched manually via the dropdown menu arrow for the item on your orders page. The tracking will update the delivered status (although it may be a bit delayed as there's currently a glitch).
eBay's new "Simple delivery" is where ebay give the buyer postage options. Details here:
https://www.ebay.co.uk/sellercentre/postage/simple-delivery?mkevt=1&mkpid=2&emsid=0&mkcid=8&bu=43952...
You can opt out of it if you like. During the listing process look for the postage options menu icon on the right of the postage section.
Just to add if a postage label was issued you can cancel that to get a refund. Go to orders, select postage labels on the left-hand side, then go to the actions dropdown menu next to the label and select Void Label.
And if you have any problems changing the tracking number contact ebay and they will do it for you. Use callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
