Agreed that inserts is the only term I've heard. The New Scientist gets you both ways. It has one stapled in and a loose one. Interesting thing is not only do they vary in their subscription offer from week to week (sometimes), but now and again the offers differ in the same magazine. I suspect them of collecting data based on the number of respondants to each offer in order to formulate a theory on the suggestibility of their readers. No doubt the editor will read a paper on it at some point.

Tell you what tho', I think there should be a word. Its in the nature of a hook isn't it, perhaps it could be an adhook, or since hook stems from Old English hoc, maybe an adhoc?