You are probably blocked from increasing the postage price on the books and / or DVDs you have listed? These have a capped postage to avoid what used to happen, that is, previously sellers postage gouging on those items.
If a higher postage cost is needed here, you have 2 choices.
Either use the full cap and add the difference to the starting price of the item.
Or, offer ' Free Postage', and add the full cost of the postage to the starting price, either way you are then not out of pocket.
Sorry, I do not use eBay's postage labels I buy over the counter postage at my local little Post Office.
Though others I have read are having issues too, with the cost of Large letters and the weights involved, obviously an issue eBay need to take up. May take a while !