at least for USPS, there are some extensions on Magento Connect that you could use.
If you need further customization, I think you will need to code them by yourself. There are two main possibilites:
1. Move the PDF-stuff to local folder
Copy the files from app/code/core/Mage/Sales/Model/Pdf to app/code/local/Mage/Sales/Model/Pdf. Thus the files in your local directory will be used by Magento, and you can also safely edit them without losing your changes to updates. Magento uses the Zend Framework's PDF class to print the shipping forms. Working with for the first time is a pain, but you'll work it out (and google knows everything).
Advantage: fast to code
Disadvantage: you will edit the only shipping form you have
2. Write a custom extension
Don't copy the files, but extend the classes und functions in an own module. This way, you will be able to create multiple shipping form designs; of course, it will take more time/effort than only copying the built-in one.
Ask if you need more advise.
Regards Simon