I find most of the comments here to be quite pessimistic, so here's a few things to consider:
There are proven economies of scale here. With any volume, a bulk postage rate will start saving you money. There's no reason to think Paul will continue to pay 44 cents a letter. Furthermore, the cost of printing is being grossly exaggerated: I picked up a laser printer from Dell that included a 1500 page toner cartridge. That's .08 cents a page, with a free printer. Its easy to do get cheaper than that per page with a more expensive printer.
Also, all of this can be automated. I share an office with an attorney who sends out hundreds of letters per day, and it takes about 2 hours of his assistant's time to setup the Mail Merge in Word and start the machines. All stuffing, labeling, stamping, etc. is done by the machines.
Here's a tip I picked up from them: if you can mail every few days instead of every day, you can get a nice break on your rate. I know you gaurantee a 24 hour turnaround now, but something to consider.
Paul's found customers and profitability from day 1, and he can just work hard until he generates enough cash to buy machines to handle it for him. It wouldn't be hard to completely automate the entire thing. I like it.
Feedback on your website: you knocked it out of the park. The domain is obvious and makes sense, the website is to the point and immediately conveys the benefit. There's no loose ends, from the privacy policy to the API. I'm seriously impressed.