I don't have experience with IAP or the object itself, but maybe these links could help:
scirra.com/tutorials/645/in ... njs/page-3
If not, maybe add a global variable and set it to 2. If amount < global variable => you can purchase. If amount = 1, set amount to 0. Something like this, maybe? I don't know wether the item would still be available if amount = 0, as i've said i don't have experience with IAP or the object, so excuse me if i'm wrong.