Back around 1969 I saw an advertisement in the back pages of Popular Mechanics Magazine that read something like this ....
BUSINESS SECRET TO MAKING MONEY FROM HOME.
VERY SMALL INVESTMENT , WRITTEN MONEY-BACK GUARANTEE
SEND $1 FOR COMPLETE INFO .
As a naive teenager I sent the dollar .... the instructions sent to me were to put an identical advertisement in my local publications and soon dollar bills would be arriving in my mail too.
The (clever) written guarantee was the fact that I had sent them a dollar which proved that it worked.
.