Also, if you're in a pinch and you use a Windows computer & have postage stamps (or can go buy them), you can use "Click-N-Ship for Business" (formerly the USPS Shipping Assistant). Up to 3 oz ships for $1.95 including free delivery confirmation--it's more than PP but less than paying at the counter.
If your packages are under 13 oz, & you have the right postage you cab affix stamps to the package and drop in any box, over 13 oz with stamps needs to be taken to the PO:
International First-Class Mail: Print Labels From Your Computer
http://www.gotogreatpanes.com/blog/2008/02/20/international-first-class-mailprint-labels-from-your-c...(It works for all services of mail, not just International First-Class--that's just the name of the blog post.)