Dave Bonta's short poem works like a small piece of economics turned into philosophy. The title sets up the idea that free thought has a price, and the first two lines spell it out: if you give up the comfort of absolute certainty, you end up paying "interest on doubt." Doubt behaves like a loan that never quite gets paid off. It keeps costing you, a little at a time, for as long as you stay open-minded.
The last two lines shift from the abstract to something personal and almost playful. The speaker writes a letter, in ink, to "the thief that stole a laugh." It's hard to say exactly who or what that thief is, but the poem suggests that questioning things can rob you of easy, unthinking pleasures, such as a laugh that comes without second thoughts. The mood is wry and thoughtful rather than bitter. The speaker seems to accept the trade, though not without a gentle complaint, and the handwritten letter adds a touch of humor and tenderness.