It is not possible to directly buy things on Amazon with BTC. If you have to make a purchase you can do it either by Amazon Gift Card or via Credit/Debit cards. There is a site that helps you to make purchase on Amazon using BTC. The site is called PurseIO. Upload your amazn wishlist on this site and deposit the cost price in BTC on the site. Other users from the site will buy the product for you and send it to your address.