[nfriedly]

Usually around $0.25 + 2-5% depending on how good of a deal they negotiated with their payment processor.

[update] Here's Amazon's fees for their SimplePay service:

For Transactions >= $10:

    * 2.9% + $0.30 for all transactions

Volume Discounts

    * 2.5% + $.30 for all transactions for monthly payment volume from $3k-$10k
    * 2.2% + $.30 for all transactions for monthly payment volume from $10k - $100k
    * 1.9% + $.30 for all transactions for monthly payment volume over $100k

For Transactions < $10:

   * 5.0% + $0.05 for all transactions

from https://payments.amazon.com/sdui/sdui/about?nodeId=6022