I wanted something like this a few years back for small scale billing. I decided to partner with a local print shop who was able to get me a good pricing on 2 color prints and envelops. I never was able to find a place with a good API that handled low quantity orders with a reasonable price. ($0.45 a bill)
Sendwrite is too expensive (obviously has a different target audience). I had to sign up to get prices.
They give a quick order of magnitude on their front page without signing up: https://sendwrite.com/ mentions "as low as $1.99 per card in a pack of 20".
$0.45 per bill? Surely that wasn't the shipped price you were looking for - stamps alone are 0.44 each... On top of that, it's a very labor intensive service.
Metered mail costs significantly less than stamped mail. A quick check with the USPS suggests a price of $0.237 per piece. That makes $0.45 per piece pretty reasonable for a fully automated service, assuming a human never needs to touch individual pieces of mail.
Fair point about metered stuff. I'd assume that it'd be extremely hard to automate this unless it was constrained to one or a couple of types of mail, though.
Sendwrite is too expensive (obviously has a different target audience). I had to sign up to get prices.