Amazon WebPay FAQ:
there is no charge to send money using Amazon WebPay.
There are no fees charged for withdrawing funds from your Amazon Payments account
Amazon Payments Fees & Account Limits:
Personal Accounts may receive up to $500 per month. The receiving limit may be raised or removed entirely once a credit card and verified bank account have been registered in the account.
The only fees I see are for using Amazon Simple Pay or Checkout by Amazon. It seems that both Personal and Business accounts can use those services. But if you just use WebPay or TextPayMe, it still appears that there is no fee. Again, I could be wrong. Can you confirm that you didn't use ASP or CbA?