It certainly does say that, and I made that point myself to someone else many months ago, when it was just starting, with only a few people having to use it [not me] and generally not many people understanding how it worked (and possibly ebay still making some changes to it.)

Even so, it's implemented as large letter - Click View listings next to my name and you will see a cased CD for sale with custom postage (with the correct dimensions for large letter).

(As we have both said, if its over 100g it can't be opted out.)