Just a note on using low or no-balance or cash-loaded cards:
Some credit card companies, like Capitol One for instance, require a re-authorization if the total amount of the charge changes in any way. The original authorization, made when you placed the order, isn't dropped for several days or weeks until it expires.
In practice, what that means is that if you only have a balance that covers your order, the re-authorization will fail and the merchant will not be able to close the transaction.
Here's an example of how that works: You have a pre-loaded card with $125 on it and your order totals $120, including shipping charges. The merchant is authorized to charge you $120 when she closes the transaction (usually when the order is shipped).
Then, when the merchant does the final shipping cost calculation, your total charge turns out to be $115. When the merchant goes to close the transaction (capture the funds) the transaction fails because the total amount to be charged has changed, and the card company considers this a "new" transaction, requiring a new authorization. Since you only have $5 remaining on your card balance (even though the original $120 is still there) the transaction fails.
So, if you're using pre-loaded or cash deposit cards (or even regular credit cards where your balance is close to max), you should check to see if this is the case with your card, and if so, always keep double the amount of the transaction available, if there is any chance that your order total will be adjusted.