To get combined shipping, just put the books you want into your Cart and then select Request Total From Seller at check-out. I've learned this button does not appear when using the eBay app while shopping with a smartphone. You need to use a PC or (if using a smartphone is your only option), go to eBay.com on the web, scroll down to the bottom of the site, and switch to Classic eBay site. Then this button will appear at check-out.
Thank you for your consideration!