Two postage/mailing services I've used are:
I used SimplyPostage -aka- Neopost for several years, the one thing
I didn't like was that you had to pay for the software updates when
there's a increase in rates, but you don't need to access your
computer for 1st class mail, you can just print from your scale.
Stamps.com is the service I'm currently using, it's a few dollars
cheaper, but you must be connected, the service is online only!
However, you get free delivery confirmation, which
normally cost you around .45 per mailing.