just a heads up that Amazon gift cards can not be used against "Amazon Payments", so you cannot buy algorithms with gift cards on the desktop version of H9 control… I just tried and my credit card was charged, my balance of credit on Amazon remained unchanged.
so looks like you will have to go down the iTunes gift card route if you want algorithms for your birthdays etc.
Amazon Gift Cards – "Not redeemable with other merchants using Amazon Payments."
not sure if this is because Eventide are registered with Amazon USA and my gift cards are Amazon UK…. But my Credit Cards registered with Amazon UK were offered as the only payment option.