@nina_kali_nina @jmax AFAIK there aren't outright prohibitions on sending cash overseas in most countries, but it does have to be declared on customs paperwork and may be taxed (in the US the limit before this kicks in is $10K and even applies to e.g. the contents of your wallet on a flight)
Domestically it's not done often with the local currency, because if it doesn't show up you're out of luck, but it's sometimes done for special reasons.
If i need foreign currency and my local bank branch doesn't have any on hand, i can order it through the website. The corresponding amount in USD is withdrawn from my account and they send me a FedEx envelope full of euros, CAD, or whatever I asked for.
Why fedex and not postal service? No idea. Possible there's regulatory issues around it, maybe FedEx has better delivery guarantees or offers a volume discount or something.