With PO the amount you pay really depends who the postal employee is. Slacker is good, stickler for rules bad.
From my understanding and the shock I got Monday, when shipping international if a bubble mailer is rigid it has to be shipped as a package - mailed 3 oz to Canada Monday cost $7+, Sunday would have been $3.30. With a card you want the package to be at least top loader rigid - some PO workers will hit you for rigid with a top loader and others will not. Just have to find the slacker - remember in the post office world slacker is good & stickler bad.