It's frustrating, and you are not alone, many are finding they are just not getting the option for Custom Postage, for items clearly exempt from Simple Delivery.
A member has found a workaround.
From the link below, bringing you to a previous thread, there Go to Solution and Rick's answer solves the oversize issue, but should work equally well for light items.
Give it a go, hopefully it will work for you, until the issue is solved for everyone.
https://community.ebay.co.uk/t5/Seller-Central/how-do-you-list-anything-bigger-than-61cm/m-p/7882638...
@tommytoo147