Send the manufacturer an invoice for the font license along with your documentation. If they don’t pay then pass the bill on to a collection agency.

I have a friend that happened the exact same thing to. It was a large chain of clothing stores known for their lax relationship to intellectual property. The typeface designer contacted the company by email with the proof and was then contacted to negotiate a compensation. He got some thousand dollars for the one-time use. It was a US company, and they are so afraid of litigation (because of the cost) that they will move to have the issue settled quickly. Good luck!

How do you know they didn't buy it? Or the designer didn't buy it?

Consider just ignoring it as an option. Is pursuing this really going to help you?

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I see my fonts on shirts all the time. Isn't that why we make fonts? So designers can use them on their products? I would take it as a compliment personally.

I realized that your fonts are public domain and not commercial. Then bag what I said in the last post.

I realized that your fonts are public domain and not commercial. Then bag what I said in the last post.

I can't see how those two statements fit together, a thing can either be a public domain or non-commercial, but not both.

I hope it wasn't this one because that would be sort of ironic. http://www.dafont.com/fr/diskoboll.font
Using my font that's free for commercial use to make a font that's not free for commercial use.

As a former font maker I have experienced many pirates in the font business. right now the biggest offender is URW++ and they have been notified.

How do you know they didn't buy it? Or the designer didn't buy it?

Probably because nobody bought it. And if the stolen font is as ugly as Diskoboll, the thief should be punished for his bad sense of taste.