How to pay Amazon.com purchase by combining debit card(s) and credit card as payment method

I got 2 $10 debit card as rebate from 2 different rebate inquiry.  One card was from buying a Hitachi harddisk, and one card from buying Patriot SD card.  The thing that I don't like these debt cards is that it's hard to combine payment with another credit card when shopping online.  Since I was about to buy something from amazon.com anyway, so I used the following process to allow me to roll these $20 credit combine with my credit card to complete the purchase, which basically use gift card as the temp "buffer".

- Use Hitachi Visa debt card to buy $10 Amazon gift card.
- Apply the gift card credit to my Amazon account.
- Use Patriot Visa debt card to buy $10 Amazon gift card.
- Apply the gift card credit to my Amazon account.
- Purchase at Amazon.com.  Amazon allows me to combine gift card credit with my credit card.

Mission accomplished.  And I am going to throw away these 2 debit cards 2 months later once I make sure that there's no leftover credit.