Make sure it hasn't defaulted to include RM on here:
https://www.ebay.co.uk/ship/prf

Depending on the category/sub category you may be able to opt out of Simple Delivery altogether and buy your own postage.  After selecting category and entering details and item title scroll down and click on the box under the heading Package Size.   At the very bottom of the pop-up you should see:
Oversized/ .... Simple Delivery is unavailable for this item
Then click on Add Primary Service to buy your own postage.   You may have to explore other categories to get that.