I'm not saying you are, just that it makes it sound like some kind of scam.
I think you should charge a low flat fee. $15 perhaps. I wouldn't be surprised if you had more people using the service this way.
We initially offered returns for a flat $10, but being new in this space, we found it was difficult to get attention and that many were still doing their taxes on paper because of the perception that they had to pay. We're hoping we can convince many of these paper filers to switch knowing that payment is optional, not mandatory.
Show me some fancy logos of why I should trust you. At the very top of the page.
Trust is your biggest problem at the moment in my opinion. Identity fraud is so widespread nowadays people are careful.