I ship internationally quite often and have uploaded the sql file of countries from http://www.ubercart.org/forum/internationalization/347/country_and_zone_... so users can choose their respective countries correctly.
The USPS quotes appear to only work for US addresses? Can you verify for me whether or not this is the case. I'm more than happy to try to debug it if it should be working... If it doesn't work, does the UPS module? EDIT (yes) it works, but you have to add lines to the module for worldwide expedited shipping as indicated in another forum post.
I've looked into this more and found a few things which to me would indicate that some changes need to be made to the USPS module to get international shipping working:
1) There seems to be some new API indications that would go into the services list. I saw these on another cart site and they said to add the following ( 11=Priority Mail International
12=First Class Mail International
13=Express Mail International Flat Rate Envelope
14=Priority Mail International Flat Rate Box )
I couldn't find any verification of these within the USPS api v3, so I don't know whether these are right, but since they have it working I would guess they are....
2) The USPS api rate quote instructions seems to indicate an entirely different structure for the international rate request (which I noticed as a TODO within the module to check for this). The module, then, would require a check for non-US addresses and then to construct a different xml query. I tried to do this myself, but it's a bit over my head (I'm surprised I found this much 
.
There are examples within the following document at the bottom within the International Rate Request section - http://www.usps.com/webtools/htm/Rate-Calculators-v1-2.htm#_Toc166663656
This would be really nice functionality to have and I wish I could do more so far as coding, but I'm more of a reader than a coder unfortunately. I would be happy to test this functionality however as it is a strong requirement for my site....