Is it the hardback saint books you have on auction they wish to buy? They haven't placed a bid yet , Is £4 the price you are happy to sell the items for? Then, just revise the item to add a Buy it Now Price, or change the auction to a Buy it Now format and adjust the price if necessary.
They can buy the item immediately, and you can send them a revised invoice with a combined postage if you are happy to post the items together.
To send a revised invoice with a combined postage, go right of the item sold to More Actions, and select Send payment Details, all item will appear on the one invoice. Amend the postage there to that required, but do not click on reset. Check it is correct, and send .
If you are on the Seller Hub, you click on the item in Orders >> Awaiting Payment, then top left you will see send Invoice, follow the above instructions.
You will need a laptop / PC and not the app or mobile browser to manage the above. Chrome works well with eBay.
@fcostello12