So I went to Mint. No sign of the sale which was weird. Then I went to LJ archive (this tool has been oh so handy over the years because it has the best search function of anything) and found that I had bought the machine on March 11. Back to Mint, no joy. Nothing in my my Capital One credit card archive or Discover. Then I remembered my old Chase credit card. Unhid that from Mint and VOILA there was the charge. But, that's not really good proof. And, I no longer have access to that Chase account.
But... didn't I download some stuff? Dropcam had a folder called finances... In that folder are PDF's of Chase 2015 plus all the months in 2016 that had transactions INCLUDING the statement I actually needed!!
Yahoo! I now have proof of purchase printing out.
I'm quite proud of myself. For saving, for remembering, for finding!