You should still be able to cancel the order citing Buyer Requests to cancel. Click on view order details top left of the sold item, then far right from More Actions >> Cancel Order.
If this is not showing on the app or mobile browser, switch to a laptop / PC and use Chrome, try from there.
To cancel an unused postage label , go to "My eBay" or the Seller Hub, select Orders >> "Postage Labels", find the relevant label, and choose "Void Label" from the "Actions" menu. You need to do this within 14 days.
@yate-ferdi