This is what I got from USPS:
“…if you need to track your International Mail please have the US senders consider using our Global Express Guaranteed or Express Mail International services.  Delivery information is available to most countries for C-series customs label numbers that come with Regular Priority Mail International, or the Medium and Large Flat Rate boxes.  You can also receive confirmation of delivery when using our Registered Mail service in conjunction with the specific class of mail you use.”
I’m sure this information about “nature of international shipping” could have been provided to you in any Post office. And please, don’t promise to send me anything by mail. Those packages tend to get lost, remember? Now I insist on a refund.