There are different Canada Post services for the U.S. than for the rest of the world so you have to separate the U.S. on your selling form.
Under international shipping click on choose custom location then check off United States. Then fill in the shipping service that you will be using. If you are using flat rate, fill in the price.
Below that you will see "Offer Additional Service"
Click on that and then on either worldwide or custom location. If you choose custom you have to check off the countries. Then fill in the rest of the info.
If you are using light packet you do need to split up the U.S. from other countries but there is no need to use calculated shipping since there is just one price for each weight tier for light packet USA and then just one price for each weight tier for light packet international which covers the rest of the world.
A suggestion... since you are using light packet for the U.S., why not use lettermail for within Canada? The price would be much more reasonable.