Mystery

I received some mystery mail yesterday.  Why is it a mystery?  Well, sadly the envelope was empty - so someone either made the effort to post me just an envelope (weird) or whatever contents of the envelope unfortunately got lost in the mail.  And I don't know who posted this.  I don't recognize the handwriting and I can't read the postmark.  I thought it might be a late birthday card, but it looks like a standard envelope.  And the envelope didn't seem obviously tampered with, so I'm altogether confused by this.  Any suggestions?  It was posted using 2 large 2nd class stamps and a note at the top of the envelope to say that these stamps cost more than the international standard post rate.  But this someone didn't include USA at the bottom of my address...

Generally, I'm not particularly enamored with snail mail right now.  One birthday card from a family member has not arrived, and neither has a package from another.  Something else to blame on the pandemic...