It sounds like the IRS didn't think the $30 was enough to cover the penalties and interest for your late estimated tax payment. Sometimes, the letters they send out are an exercise in obscurity.
Of course, the other possibility is they mad a mistake. I got assessed a penalty for late payment when I had filed an extension with what ultimately turned out to be the correct payment amount.
Despite having signed the return receipt (green) postcard and cashing the check, they lost the filing for extension. Eventually, they closed the matter, but their reason was: "you've always paid on time, and we are granting you this one-time forgiveness".
I couldn't make up a more absurd story.
“I think you should read the letter closer.”
As I was sending it to my accountant I did just that. The first page summary made it seem like what I had said. But, the other pages of it showed the actuals. The $30 penalty was the standard rate I think for paying late. Then a penalty based on how late it was. And then interest on the tax after it was late (58 days).
Sometimes it is a real drag being stupid! Like John Wayne said “Life’s tough. It’s even tougher when you’re stupid.”
Of course now I’m wondering why my accountant didn’t figure it out properly?