Unless your "downtown post office" is a BMEU (Business Mail Entry Unit), you will not be able to enter your mail there.
The rules and regulations for bulk mailings is so complicated, that, it has taken many of us years to master. You might want to take a quick gander at the "DMM" (Domestic Mail Manual) you can find it at
Postal Explorer. In this book you will find regulations relative to your mail piece design, (aspect ratio, background reflectance levels, minimum post card thickness, etc, as well as addressing regulations, and mail prepartion regulations. There are also regulations governing your mail list (CASS, NCOA, etc).
In addition to your printing and cutting equipment, you will need to invest in a postal sortation software product, (around $8,000/year with updates), an NCOA data service (we pay about $2,000/year for unlimited NCOA runs) as well as your creative software license (Adobe InDesign, Creative Suite, etc). Then, unless you want to wait hours when sending a job to your printer, you will need to invest in some variable-data merge software. You can get by relatively cheap there if you are not going to do anything big or complicated (FusionPro - around $1,000 plus support). We do a lot of large complicated variable image, variable data jobs so, our merge software is XMPie Svr version ($100,000 +) with an annual support contract of about $12,500/year.
If you're only paying $0.40/ech including postage, I'd say you are getting a pretty good deal. In my humble opinion, if you invested in everything you need to be a small print/mail shop, AND figured out all the rules and regulations (which, by the way, are changing constantly, so, you have to always be on the lookout for new regs and implement them by the deadline date, or your jobs will be rejected at the BMEU), at your volume, it would be years before you would hit a breakeven point on your investment.
Also, "postal account reps", really know very little about the mail business. They know what to do when a mail job hits their dock, but, in reality, none of them have ever actually "prepared" a mail job from a production point of view. Also, by law your postal rep can not "recommend a postal sortation software vendor". He can provide you a list of USPS Certified CASS and Postal Sort Vendors, but, he could lose his job if he actually recommends one over the other.
I really wish you all the luck,
-Best
MailGuru