Solution: I first tried entropay.com and it didn't work for me. but the following way worked for me:
- I bought an amazon gift card for myself.
- I created a new amazon account and registered my kindle.
- I redeemed my gift card in my new amazon account.
- I made sure no credit cards are saved in my new account and the default payment method is not set for my kindle.
- I was able then to buy books :)))))))) Amazon will check the gift card balance if no credit card is set for the kindle.