Good point Dave. I'd never done it before, so I wasn't aware. The fee I was quoted was $22.95 for a payment of ~2600. But the in the final step I was also charged a "large payment security fee" of another $40+, bringing the total % to about 2.5% The website describes the fee this way:

<b>What is a large payment security fee?</b>
For larger than standard payments, there is a higher incidence of fraud. We purchase insurance to cover this risk. The large payment security fee is the insurance premium required to submit these payments in the most safe, secure way possible.

Note that you can get $10 off the fee by making it an automatic payment.