What happens if you put zip code where it says postal code?

ZIP code and postal code are the same thing. They are called ZIP codes in the United States because

ZIP is an acronym for Zone Improvement Plan, proposed in 1944 by Robert Moon, a postal employee, and finally implemented in 1963 (on April 30, 1963, Postmaster General John A. Gronouski announced that the ZIP Code would begin on July 1, 1963).

The post office came up with a cartoon, Mr. ZIP, better known as Zippy, to promote the.use of the ZIP code because people and companies would forget to use it.

I send mail to the US almost daily and I have never used a sticker that said "Ship to USA". And as for Cdn postal workers, they really don't need to know where they are although I'm sure they do. With the volume of mail going from Canada to other points around the world ... they know. But even if they didn't know (or didn't care) the mail would still go through. Just the fact that it is going to the US is all they need to know. When they see USA or even a US zip code they don't sort it to the various States/locations. It just goes to a central clearing location for shipment to the US and from there YOUR postal service sorts it and sends it on it's merry way. Its the same way your mail comes here. If someone doesn't use USA or the zip code on it, or Canada and the postal code, then someone reads the conventional way and sends it on to where it is supposed to go or returns it to the sender for proper addressing. If, as you say, the piece of mail is that important then it's up to the sender to ensure that it's addressed properly.

your postal code is your zip code