You may have opted into Simple Delivery and need to opt out, to be able to select the postal services and price you wish to use.
Simple Delivery is not compulsory..... yet, so you can opt out if you wish. I believe you'll need a laptop / PC and use Chrome for this, maybe not the app or mobile browser.
When creating a new listing here's how to opt out:
Click on Collection then straight away back to and click on Postage Only, and the Custom option shows, you are then able to select the postage service and cost you wish to use.
Or, if there is a switch link, click on ' Switch to Advanced Options' at the bottom of the postage section, and once again when a pop up appears.
@leete6335
