Have you noticed that nowadays, Amazon consumers would not care much about the package inserts ? In my last product, I had an insert in the package inviting the customer to join a VIP club for future deals notifications. None of them signed up for the VIP club after 500 sales. So, my feeling is that, to capture emails :
* VIP Club : Does not seems to work
* Warranty Registration, may be ? may be not ? Because the value of the product is not so high ?
* Free Gift (ie., eBook), may be ?