Simple, you just say, "Thank you for your generous offer, but I have one in the trunk of my car already." See, if you read enough etiquette columnists, you begin to get the hang of this kind of thing.

Also, it suddenly strikes me that if you know about the kirin and the swear-to-never-desert-my-post-before-your-throne bit in advance, you lose out on the awesome WTF aspect of the "Uh, Yoko, I think you're the queen of Kei" scene, and the whole novel might read differently. I enjoyed being as surprised and flabbergasted as Yoko about that.