Don't know if this will help you sellers out at all BUT I paid for 4 items from the same seller yesterday (19th December) and had trouble checking out with paypal when I tried to add a message to the Seller. 

Ebay has changed the way checkout is processed, I do not know if everyone has the new checkout or just some.  I also got asked to give feedback on what I thought of their new checkout procedure.

I gave them what for, because I am tired of them changing everything all the time, as in now you have this feature, now you don't.  Let's try this, no let's revert back to the old way.

Anyway, this may well be why buyers are able to edit the postage costs on their items regardless of a sellers settings.

Just a thought, but if more than 1 seller is experiencing this problem, it could well be the answer.