I just came to the realization as I'm updating my Canada Post rate sheet for 2015 that I've been paying the wrong postage for almost a year!
I guess when the lettermail rates changed in March, I forgot to change them. So as far as lettermail goes (international, US, and Candian) I have been underpaying quite significantly since March! I mail out a LOT of stuff and I use postage stamps. I haven't had a single item come back to me as 'Postage Owed'.
I'm not complaining at all, but what the heck??? 🙂