If they are all on the same ID, tell your buyer to purchase them THEN WAIT FOR YOU TO SEND AN INVOICE.

Send the invoice (all books will be grouped already) for the agreed price. Whilst you can't discount the headline price, you can adjust postage to reflect the agreed total price.

If you have 'free' postage, the only way is to end the individual listings and list the set together.

If you do that, I suggest listing as BIN with best offer, and set the BIN way above what the books are worth. Message the buyer the new item number, tell them to make the agreed (not counting postage) offer and only accept their offer.

And hope they go through with the deal.