Yes, we do. This helps us defray the costs of developing and running the Payburner system. Our monthly server bills are currently north of 300 USD per month.
The fee is 1% of the XRP amount of the payment. Payburner keeps a running total of the fees you have incurred and displays them in the Pay Button Manager in the browser extension.
In this image you can see that the SpudStore's running total of fees is current -0.016721 XRP. To pay your fees balance, simply click on the fees button (the orange one showing the balance), a payment form appears allowing you to make a payment to PayButtons$payburner.com for the amount of fees due. You can optionally increase this amount and carry a positive fees balance. We do not support positive fees reimbursement at this time.
Once the payment has settled, you can re-open the extension and you will see that your new balance is zero.