Hey - congrats! I wrote an entire business plan around a very similar concept 10 years ago! Never executed on it, but still think about it all the time. Since I know this is too big of a thing for me to tackle, let me share some of my features with you that I think (hope) can help.
1. Assuming you've purchased the equipment to do mass production (i.e. automated print fulfillment center with folding/cutting/licking/stamping/etc... features then make sure you're doing address cleansing for the extra postage discounts.
2. Make a utility print driver so I can "Print To Postal Shout" -- this is VERY easy to do, at least on windows. You can get freeware, or near freeware, to act as the actual postscript or pdf driver and it will alert your application when the file is done printing - you can then kick in and do something meaningful with it (i.e. submit to your service). I've written such print wizards in the past and would be happy to elaborate if you're interested.
3. In combination with number two above, this is where you can expand your reach considerably. Forget about that template that I have to download. Through the print wizard, you pick your addresses and contacts from a list of contacts, or start adding them. Since your program is controlling the wizard, you can get the addresses in the format you want. Upon completion, create an XML doc (or similar) and call a web service or ftp service to submit the document(s) to. You can either encode the actual document into the metadata of the xml file (slower) or give it a GUID and point to it from the XML metadata. I highly recommend an FTP based approach for file transfer as it is fast and you can give your customer progress feedback or even sync in the background.
4. When using the driver, ask the user HOW the document/message should be delivered. They will build an address book over time (maybe you can even tap into an existing CRM if they have it) and offer email/fax/postal/premium). Think of this from people who want to send an email to 100 people, but only 95 of them have email addresses (hard to imagine these days, but used to be very common). Hope this makes sense. Further, it's a way of offering MULTIPLE delivery methods in one easy wizard. You get to charge for the fax and postal and maybe offer the email free. For example, I want my lawyer to see the document I need reviewed right away, but I also want to send hard copy to him and cc myself.
5. If you could somehow incorporate electronic signature / notary / SOMETHING into the documents and build up a crowdsourced delivery mechanism that would be completely disruptive. Meaning, I'm in California and I want to overnight this signed document to New York. I could send it to your service and have its authenticity verified, then you can reach out to your New York based notary/courier and have it delivered within a few hours to the New York address. Way cheaper than FedEx and if you can somehow get this implemented it would cause your service to absolutely explode in usage. Plus, it would be more environmentally friendly! :-)
6. Offer stationary / letterhead for the customers to design and use. This could be a real problem for mass printing though. At minimum, you should offer different qualities of paper for it to be printed upon. This is a "nice to have" and nothing I would focus on right away (IMHO).
7. You could offer discounts on the actual costs if the user was willing to have ads printed at the bottom or top (or inline) in the document. This wouldn't be for official correspondence, but more for the casual messages you might not otherwise mail, but since it's free, or near free, why not. The beauty of this: It WILL get opened and read by the recipient. Here you have a personalized letter sent to someone with local adverts printed directly on it. The recipient is almost guaranteed to read it and see the advert.
These are off the top of my head. I could drum up that old business plan / feature spec and give you more details if you want.
Hope these ideas are useful to you. As proven: Ideas are useless w/o execution - bad on me for not doing this myself.
EDIT: P.S. - target Salesforce.com as a market because this is especially useful for mobile sales reps who have to fax and mail from hotels all the time.