Are there different rules in Canada regarding this? There aren't any laws here in the US that make it illegal to mail cash. On the other hand, it generally isn't a good idea...
snopes has an article about this for the US mail system:
1968 400 Coupe, verdoro green, black vinyl top, auto 1969 GTO Judge coupe, Carousel Red, manual 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver), Fire Red, white top, auto 1972 Buick GS Stage 1, Royal Blue, black vinyl top, auto