I did send more than $3 in mint US stamps, recent issues, He apparently has chosen not to re-rate. They should have been received.. mm, a couple of weeks ago. And I made sure the contents were flat, flat, flat, lol.
When at all in doubt, I've started measuring thickness of the envelope!
My local mail carrier and post office couldn't answer the question as to 'why' someone decided it needed $3 additional postage. They know I'm careful about attaching correct postage so they too were surprised. Without having the actual envelope as it was sent, contents included, there's no way to know. All we know is someone decided it was a package.
Thanks for all the input and suggestions everyone.
Time to put this matter to rest I think.
Stamps in the US are valid until used, regardless of age. And that would have been just 50 cents (half a dollar) not $3. And as long as the denominations add up to adequate postage, there's no problem using various denominations, even if they say 'additional once' or 'two ounce' or 'non-machinable'.