That's what I was wondering -- Tarsnap may be an unusual case, but out of over 10000 transactions I've only had one chargeback... and I won that because it was initiated by accident.

Agreed. CircleCI has had one chargeback ever. It was $19 and by a long-term customer who didn't do a chargeback for any other month. We assumed it was an accident, but didn't contest it (not worth it).

Why not ask your customer why they did the chargeback?

It could help identify some hidden problem and as a bonus make them cancel that chargeback.

I think it looked like an accounting department had made the chargeback, which implied an accident. It could have been a hidden problem but it didn't strike us as the highest priority one (we've plenty of problems people are telling us about that go higher in the queue :))