this is a major thorn in my side.
I run www.expataudio.com
with another designer. I am the designated shipper. (joy!)
I set up our online store (using cubecart v3) to only do USPS Flat Rate shipping for customers in the USA. (I'm based in Texas).
That's quite nice, because I can go straight to paypal, and print the label immediately.
However, 70% of my business is from Europe and Asia... which means customs forms... and lots of customers picking first class mail options. (I can't filter international shipping options).
Today, I have to copy/paste each delivery address, describe the contents of the order into an excel template, then run a USPS Customs Form Firefox plugin that generates all the forms for that batch of shipping. Its a slow, painful process.
It would be much better/nicer if I could simply hit "print shipping and customs".
Also - while I'm whining... I looked into Amazon fulfillment. They are great if you are only shipping in the USA. They won't do the customs bit :(