The network effect of mass messaging was tested at smaller scales many times. Victorian era pneumatic tubes come to mind or unlimited local calls later, but there must be countless other examples that did not require changing the economy as a whole.
I think the overwhelming majority of transactions can be characterized as a trade (of goods for money, services for
money, work for money, money for other money (in form or time), etc.)
UBI seems quite different in that regard and, while it doesn’t invalidate everything, it introduces a lot more of money for nothing and a corresponding nothing for money trade that is required to fund it.
Courier services have existed for a very, very long time. Heck, Ben Franklin was running one to help connect the 13 colonies.
Presumably the government could have actuslly piloted the USPS if they wanted to, only supporting the northeast for example. They just didn't need to, the leap from private courier systems to a government run courier service didn't have many unknowns to test out first.
It was obviously valuable and workable from the smallest scale possible: a verbal message carried by a runner.