I sent a handwritten thank you card after one interview, even bought nice paper and everything, and I'm pretty sure the guy used it as a coaster. What really sticks with me is another interview where I asked why their reports kept breaking and the manager talked to me for 20 minutes about it. Got the offer two days later. Now I just ask a real question at the end and skip the card. Your mileage may vary, some older managers still love the notes. But a good question seems to do way more work than fancy paper.

Wait, did Dave actually make you buy him lunch or did you just offer out of guilt? Because honestly the real lesson here is that clip-ons probably cost less than a month of buying lunch for the guy who was right. Plus now you get to be the smug one next time somebody shows up to a group ride with no fenders and a soaked backpack.