You should still be able to opt out of Simple Delivery during the listing/revising process. 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 try clicking on Collection Only, then click back to Postage Only and Custom Options should appear, which is what you want. Or remove the pre-entered postage weights, see if that helps.
@stevebonsaitree
