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Nope, you are right - apparently USPS now does have real tracking to some international locations.
I hope they expand that list and bring back the insurance on flat rate boxes.

Yes, I know, but those tracking numbers are not guaranteed to work because they are not real tracking numbers, just a courtesy scans of the customs label. So, if the postal service in US or abroad...

We've talked about this before - this is a 'courtesy' tracking and while it may work in many (or for you in almost all) cases it's not guaranteed to work because it's not a tracking number.
Seller...

Or if the 'tracking' number which is really the customs declaration number and not officially recognized as tracking by USPS, doesn't show any activity.

If the buyer is reasonable they recognize...