Compare to leaving a platter of bagels with a card asking people to pay if they can.

People are not curious if they can take a bagel for free - it's obvious that they can. So people are not tempted to do it just for the experiment.

And people wanting to pay can use cash, so the second situation doesn't apply either.

Most real world situations are more like this case than the cards case.

That answered my question, thanks. I was thinking of larger-scale, internet efforts (In Rainbows, Louis CK) but of course those don't involve physical delivery. Had not even considered honesty boxes.

Thinking more, I do remember reading about someone who started a business in DC delivering muffin baskets to beltway-area office buildings. Their business was purely honesty based, and I think losses due to cheapskatery were around 5%. In the end, they'd cut off buildings with rates worse than that. All that is more support for your points.