That is correct , Simple Delivery is mandatory for all Private sellers, this started from April 15th and some sellers still lucky to date to be able to use Custom Postage thought wrongly that it was still a choice. It's been a very slow roll out for some.
Items listed with Custom postage are being automatically changed to SD, and worth checking, as errors are being found.
There are items which are exempt from SD. Items exceeding certain dimensions (e.g., length > 120cm) or weight (e.g., > 20kg) are not eligible for Simple Delivery, here you should be able to select your postage costs.
@beatrice834