$16?! Well, I never thought it would come to this, but you’re going to have to flee the country. First, enact a scorched earth policy and have a FGM-148 Javelin Missile “Accidentally” hit your house. Then flee the country and go to Cuba. Your new identity will be Baguette Von Blitzenblerster, a banana slug farmer whose hobby is making slightly shittier duplicates of Pokémon cards. Enjoy your new life. But in all seriousness, I’d say to write a check for the amount, plus about two dollars extra(in case of late fees), and mail it and a letter stating your situation to the IRS. Since this is probably the only thing wrong you’ve done, they probably understand the situation and cut you some slack.

As others have said; always save evidence for years. Pretty sure the IRS can request your financial data from up to eight years back if they choose. As far as your situation goes...I'd send them an email. They probably have a help address for this sort of thing. [spoiler]Sorry, all out of salt.[/spoiler]

POV: you are OP