*Before you reply to this comment, remember. We are dealing with the USPS here. I know what I am saying make no sense. This is USPS's fault, not mine

*The last time I looked into this was 6 months ago so my info may be dated, but considering the speed at which the USPS moves, not likely.

USPS Does not have accounts like UPS or Fedex with monthly billing. You have to pay for each shipment, label, or stamp as you go. USPS has a web site similar to UPS or Fedex, but the USPS does not have a XML API which allows you to quote shipments and generate labels. You have to do this on their website or with one of their approved vendors like Endicia Dazzle. The reason they do not have an API is because "we are a government agency and so must be fair to everyone, so we only have two or three partners who can do this , and you have to buy the software from one of them."

The end result of all this cockamamie bull$%@t is that the best way to automate USPS shipments and label printing is to buy Endicia's Dazzle software. You can tell it your database address and table setup and it will automatically generate shipments, print labels, and upload the tracking number to your database. It is a little complicated to setup, but it is doable. If someone who has done it could write a little guide for Dazzle and Ubercart, that would be great. If you know of a better way, tell us. My information is mainly second hand and could certainly be wrong.
Profitprophet's post, New to Drupal, has more discussion about this in question 3b.
Peace
Andy