Just a word of warning from a backer of many Kickstarter projects. Note that shipping batteries to most countries is complex, as a lot of countries have their own special rules. Adding the option to order additional spare batteries, will add a lot of complexity to your logistics. Unless you work with an experienced logistics expert/firm, this will create most likely an additional delay as seen with many other Kickstarter projects. I would recommend to send spare batteries in a separate package and not include it with the Flipper in 1 package.