Two things:
(1) I remember back in primary school (ca. 1971) reading in
My Weekly Reader about a girl who didn't have a stamp and taped a nickel and three pennies to her envelope -- and the USPS even cancelled her coins.
(2) A lot of people don't realize this, but
Canada Post will not bother charging Postage Due on anything that is shortpaid a nickel or less, because it's not worth delaying a letter for such a small amount. Everytime the postage used to go up a penny at the first of a new year, I used to tell people not to bother buying sheets of 1 cent stamps, and to just use up their old postage. This, of course, didn't get me any friends among management, but did get me several from the public, who, I rationalized, were the reason we were in business, anyway!