Apart from hand writing my address on the prepaid "Xpresspost/USA" envelope correctly at the "TO" column,, I wrote my address at the back of the envelope as well in block letters, to be on the safe side. I found the address sticker provided by Canadapost was fairly small.
However when I received my passport back, I observed CPPO had pasted my address sticker generated and printed in their system, over my hand written address.
Hence I would conclude, it is not your mistake and hope Canada Post sends back you the package without any more delay.