Now that more post offices are catching up with the new requirement for international packages to have the customs form submitted electronically, there are lots of unhappy folks standing in line at the post office while the clerks enter the data manually...
Good news: You can print your international labels from your computer and submit the data online yourself so the clerk doesn't have to--and you don't have to stand there and wait.
Some folks use endicia or stamps-com to pay for postage and print their labels & customs forms--great if you can afford it/have enough business to warrant paying for the subscription, but many don't.
For Priority and faster services:
You can print your labels through Paypal or USPS.com's "Click n Ship" and the subscription-based web sites--they will be three pages long and you'll need the plastic envelopes USPS provides for free (get them from the PO or the USPS web site store) or any attachment to the side of your package that can be easily opened by customs in the destination country.
For First Class International:
If you have a PC, you can use the free downloadable "Click n Ship for Business" program--formerly the Shipping Assistant--or the online "USPS Web Tools" system. ("Click n Ship for Business" is different than USPS.com's "Click n Ship", I don't know why they changed the name recently, it makes if very confusing.)
If you have a Mac, you can use the online "USPS Web Tools" system.
Both the "Click n Ship for Business" program & the "USPS Web Tools" system submit the data electronically, but you'll need to use postage stamps to cover the postage so if the package is over 13 oz, you'll still need to hand it off to a postal employee to mail it (as is true for both international & domestic packages over 13 oz with only postage stamps).
More help with the free program & links to to the USPS web site here:
Clarification of New International Mail Rules
http://www.gotogreatpanes.com/blog/2012/05/22/clarification-new-international-mail-rulesMore help with domestic & international shipping labels, insurance & more here:
Shipping Basics
http://www.gotogreatpanes.com/blog/shipping