Sellers can only DIY if in certain categories, and under £20/100 gms.
I have looked at some of your listings, and you have some items using your choice if postage, but others are dictated by ebay.
There is no list of Exempt categories but it could be worth playing with the categories as sometimes there is another category that covers the item. Sometimes “other” can solve the problem.
I sell mainly badges, and find that there is sometimes a particular sort of badge (at one time sports badges were ok but not football badges) so I used sports, until ebay eventually sorted it.