The charge was for the book, and apparently it was paid to News Limited (The Australian newspaper). I didn't have quickbuy turned on, and the amount due according to Kobo was 0.00.
All I can imagine is that, because our subscription to The Australian is not current, either The Australian or else Kobo itself decided we hadn't been entitled to the free book, and took it on themselves to collect payment from our PayPal account. Since we have used this account in the past for both Kobo and News Limited, I don't know which one generated this payment, but I have emailed both of them as well as PayPal itself.
I'm not denying that I was wrong in assuming I was eligible for the free book, but I am amazed that anyone can take take money from our PayPal account without our authorisation, and without even contacting us. I'll let you know when and if this is resolved!