AH, I hadn't scrolled far enough down the page.
The problem is ebay's asinine assumption regarding how low they think shipping costs are -- there's a message saying "The total amount for shipping service cost and insurance exceeds $4.00 maximum allowed for selected category."
How does ebay expect me to ship a hardcover book and a softcover book for $4.00 INCLUDING insurance, when insurance is $1.70 minimum????
I knew about the $4.00 maximum shipping, which in itself is too low if I ship priority, but I didn't think that the insurance was supposed to be part of that amount as well!