I wish to use US currency ($1 bills specifically) because I want to lean on the counterfeiting security and laws. I wouldn't know how to verify paper rubles, pesos or rupees, or how unique each bill is. It would take huge balls for someone to knowingly counterfeit a FRN and send it through the US mail system.
I'm not disagreeing with your decision and reasoning but should point out that counterfeit FRNs were for sale on that tor market that was shut down and shipped via US mail.
But yes, a neat way to piggyback off the anti-counterfeit features that an ordinary person accepting the note can use to help verify its authenticity.