Have you looked into stamps.com? I recently signed up, they have a 4 week trial. I am hooked. I will be signing up for a $16/month service after the trial, but I'm willing to pay $16 a month for the convenience.
I do 1st Class packages, Priority, and International Air Letter & Air Parcel. I have the carrier pick up all the domestic packages at my house. I take the international packages to the post office (customs forms completed and attached) and just hand them to someone, I don't stand in line.