I think it depends on how you list them. I use Custom Postage not simple Delivery. To list with custom postage, there are several hoops you need to jump through

You have to choose "Large Letter" option - even if it isn't

You have to set a start price for the auction

Start price has to be under £20

I don't use eBay labels or Simple Delivery. It all works if you stick to the above. If you use Team Access, almost none of it works