I created a very similar app a year and a half ago: mailcongress.org

https://github.com/dockyard/mail_congress

It's since come down. Here are some things you need to consider

1.) Progressives won't respond well to this. I have some experience here, I spent a year working at the Democratic National Committee in DC on their Labs team. My observation is that unless you're "a name" in the progressive tech sector or have a name attached to the product it is going to be difficult getting notice. I actually respect the Republicans on how they adopt technology: throw everything against the wall and see what sticks. I can cite examples of this if people are interested.

2.) Since 911 Snail mail can take weeks to arrive. Everything that is sent to the Hill in DC is sent out for anthrax screening. It is very difficult to send reactionary issue snail mail unless it is hand delivered.

3.) The best way to get your rep's attention is to send mail, email, or call about a very specific issue. If you say "Support gay rights!" you'll most likely get a very well printed form response letter a few weeks later. If you say "I am one of your constituents and I want you to support SR 1992 up for vote in two weeks" this goes much further. Reps want to know that you are a vote and how you want them to vote on specific bills.

4.) Most hill staffers will actually ignore snail mail that doesn't have a postage stamp (the ones that the post office will put on the letter to mark its origin) from their district or state (for senators). Again, they really only care about votes. If you can't vote for him/her then you don't matter nearly as much as someone that can.

5.) I originally designed MailCongress because I saw a Communicating With Congress report when I was at the DNC. I cannot find the link now but this report comes out every 4 years (right after each mid-term). It represents the previous 4 years after publication of data on how congress responds to different forms of communication. The report I saw came out in 2006 which means it covered 2002 - 2006. At the time Email was way down around 30% efficacy and snail mail was up to 85% effective. I released MailCongress right before the 111th Congress left at the end of 2010. The next report came out that represented 2006 - 2010 and things changed a lot. Email went up to mid-70s efficacy and snail mail dropped to mid-70s. For the 4 years prior to 2010 email was just as effective at communicating with congress as snail mail. The report said the reasons for the swing was most likely because of the major turn over on the hill in 2006. When many new congress people come in they bring a new generation of hill staffers, younger, and more tech-savy. We had another major turn over in 2010. Which means more shift. I wouldn't be able to tell you what the numbers are but my guess is that email exceeds snail mail by now. In fact, I wouldn't be surprised if the next report's number were very skewed once you take Twitter and Facebook into consideration.

I applaud the effort, I really hope it works. Unfortunately you're fighting a lot of factors here. Your pricing is much better than mine (I was charging $1 per-piece and also would notify the senders when their letters should have arrived so they could follow up with phone calls). You should consider how the printing is going to scale if you get serious about this. I actually built out of mail queue backed by Redis. Scalability testing showed I could print 1000 letters per hour. Which is really not that many if it were to take off, any crazy issue that comes up (and they always do) can be an opportunity to get people to use this. The usage patterns in politics are very spiky so you need to be ready for immediate scale.

TL:DR; I once built something similar, best of luck! :)