eBay say that if you miss the collection you can rebook it on the Royal Mail website. I would think that applies the same if they miss the collection. Or you could let it lapse and post the books.
https://www.royalmail.com/collection
If you change the postage you would either have to buy the new label yourself. Or cancel/refund with buyer's agreement (via the dropdown menu for the item on your orders list) and relist with correct postage for them to buy again.