My main takeaway is to keep a bit of cash and an email adress in a wallet. Also, I have to wonder how much the fact that the assistants would have been perceived as foreigners in most of these countries skews the results.
Definitely a concern. We took steps to examine if there were specific "experimenter effects", that is whether response rates were affected by one research assistant rather than another, and don't find evidence that was a factor. But that doesn't address your broader concern that things might be different if the person dropping off the wallet was native to the country. FWIW the wallets were designed to signal that the owner was probably a local and not a foreigner.
Thank you for the response! I did miss the supplementary material notes.