I wonder if you have been opted into eBay's Simple Delivery?
You can opt out of this service when creating your listing, click on custom postage, this is next to, simple delivery on the right hand side, it should then allow you to select your postal choices.
Perhaps use a PC / laptop and use Chrome if this is not possible on the app.
You can read what eBay's Simple Delivery means here:
https://www.ebay.co.uk/sellercentre/postage/simple-delivery?mkevt=1&mkpid=2&emsid=0&mkcid=8&bu=43952...
@jamesbari