This is happening a lot at the moment. It's because you have not opted out of their Simple Delivery. If you revise your item you can do it now. Click on "Switch to advanced options" at the bottom of the postage section, and again on the pop-up you'll get.
If you're not getting the switch link remove the pre-entered postage weights and the link should reappear. If not click on Collection Only, then click back to Postage Only and Custom Options should appear, which is what you want.
