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
[update] Here's Amazon's fees for their SimplePay service:
For Transactions >= $10:
Volume Discounts For Transactions < $10: from https://payments.amazon.com/sdui/sdui/about?nodeId=6022