Did you edit your post? I seem to remember reading that you were planning to lower prices with scale.

If that is true and you're finding that people are price sensitive, you should list your lowest price on the front page ("as low as 4 percent!"), and make it clear in the signup process that rates are higher at lower volumes. Ten percent is not a lot to pay at lower volumes considering you take care of payment processing and tech hosting, and merchant accounts at Paypal start around $30-40 USD per month, so someone will need to be collecting at least $500 a month before they come close to breaking even.