Sorry, I can't find it either. I think they gave me misleading info. For your current sold item you would have to buy the postage yourself and change the tracking number at View Order Details > Postage > Track Package > dropdown arrow. That has to be done on a desktop browser. If it doesn't change generate the original prepaid Simple Delivery label and try again.
For future listings use seller pays postage/free delivery and do the above. When you pay for an unused Simpel Delivery label you can cancel it here (you have to use trackable replacement postage to get the refund):
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
When the buyer pays for it it will be refunded to them automatically in 30 days.
