You appear to have found the exemptions from using the stupid simple postage for your low value items, so what are you trying to list and what category does it come into?
There is currently an issue that Simple Delivery not offering Custom Postage to oversize items, lightweight items and those under £10, these are all exempt from Simple Delivery.
There is a workaround.
From the link below Go To Solution and follow the advice in Rick's answer to list your item, you can adapt it for items under £10 in value.
