Old tech geek here. The RFID discussion makes me twitch slightly.
All RFID-blocking methods (assuming the item in question actually *has* RFID and is not just a "marketing gimmick") are basically variations on a "Faraday cage," which you can replicate with a sheet of heavy-duty aluminum foil (or two layers of the regular stuff) turned into a pouch. I've used this cheap method with much success.
That being said, remote RFID attacks are few and far between — most theft via wireless chip hijacking happens at skimmers. But the most common form of theft is simple human engineering; the perp takes your card, disappears into another room or a little annex, copies the info, returns the card to you. I've also had the info copied *while I was standing at the counter,* and the employee didn't bat an eye. I found out about the theft weeks later when I got home and Mastercard alerted me to weird charges. AMEX, on the other hand, is more aggressive in their vigilance and stomps down hard on stuff that doesn't pass their smell tests, for which I've been thankful more than once.
Vigilance is fine, but don't beat yourself up too much if you've been scammed. Scammers are extremely smart and seemingly tireless, and may eventually get the best of you.
Cash is not copy-able, but of course, it too can be stolen. As my favourite economist likes to say, "There are no solutions, only trade-offs." I've settled on a trade-off I can live with, and I'm sure all of you can do the same.
Buen Camino!