You should use an exchange service between voucher and bitcoin such as purse.but you can also use zebpay as it is an india wallet and they also allow you to buy voucher.
Purse give the best discount and go for it.
Purse doesn't accept money for samsung. They only do for amazon. @op, find a trusted member on the forum and ask him to preorder it for you. Due to it being a preorder only right now, there is a risk, so I would suggest just wait till it releases and then ask someone to order it directly. By the way, the link you posted shows it to be in stock.
