Have you tried peaseblossom's instructions? 

I have had to change the package size on a simple delivery label from large letter to small parcel and it was really straightforward and no extra charge.  Much easier than the old days where I would have to fund the extra postage cost myself or ask the buyer if they wouyld mind payting extra.

I guess these changes have been designed with private sellers in mind as they are less likely to be sure of packing sizes and postage costs than business sellers.