The post office will only cancel something that has the required amount of postage. For a $2 bill, that was the regular first class rate - 13 cents. You could probably do something like that with newly issued currency, but they would look at you funny and have to consult a manager, who would read the manual and then call Washington for additional direction. Back in April 1976, though, this was an organized plan between the post office and the federal reserve to help commemorate the Bicentennial - and to get a bunch of $2 out of circulation at a substantial profit to the government!