Since October 1 of 2015 USPS stop Tracking of Import Registered Mail of some countries: https://about.usps.com/postal-bulletin/2015/pb22424/html/info_002.htm
This is the final list of countries than USPS accept registered mail for now: http://faq.usps.com/?articleId=1437076354655
I’m a seller from Spain, yep isn't is on list... now I'm having trouble with some shipments because the tracking number is useless.
Shipment (DHL, EMS,...) to the United States with valid tracking numbers costs 5 times more than a normal registered mail to other countries.
To try to change this situation I found this petition on change.org: https://www.change.org/p/usps-usps-redefine-the-tracking-policy-for-international-packages