Simple Delivery is now mandatory for all Private sellers.
You have been one of the lucky ones if you have been able to use Custom Postage on all your items up till now.
Existing Items are being automatically converted to SD postage, and yes, many are finding errors and needing to manually check and correct / edit their listings.
Custom Postage is now only available for exempt items such as items under the value of £10, under 100g in weight, and oversize items over 20kg. Anything else, only Simple Delivery is available.
@beardo46