Neither snow nor rain nor sleet nor dark of night

You should have recognized that as the postman's creed- 

"Neither snow nor rain nor heat nor gloom of night stays these couriers from the swift completion of their appointed rounds."

 Here they have added a bit nor sufficient postage.  Allow me to contrast the Canadian system as I know it to the Scottish system as I know it.

Canadian Post
If you don't put sufficient postage on an envelope it gets returned to the sender immediately.  As the sender you have defaulted the original postage as it's been "used".  You then put the correct postage on the envelope and mail it again.

Royal Mail
As explained to me by the guy at my local mail depot, so I can't guarantee this is completely accurate.  If you mail a letter here with insufficient postage it still gets sent.  The receiver gets a notice that they have a letter to pick up.  In order to pick up their letter they have to pay the difference between the postage already paid and the actual postage required.  As well there is a one pound administration fee added.  The mail depot holds the letter for 3 weeks and if it is not picked up it gets returned to the sender.  I'm not sure if then the sender would have to pay anything.  If there isn't a return address on the envelope it goes to a holding depot where they open it and attempt to find a return address that way.

I found this out when I was picking up a package from the depot and the gentleman ahead of me had to pay a 1.10 pounds for a birthday card.