Hi alymars,
All imports into Canada by mail are technically charged Customs duties if they are valued over $20, or over $60 if marked as a gift.
Because Customs check mail randomly, it's possible that some shipments would slip through without being charged, which is why not all packages will get charged.
See here for more info:
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Basically, you can either:
(a) Place a note in your listing saying customers are responsible for all customs duties (and GST also applies in Canada too on imports).
OR
(b) Some sellers send packages as 'gifts', although this is NOT legal.
How much duty is charged depends on the classification of the goods you are sending. It's best to ring Customs to find out how DVDs and your other merchandise is classified. The code will then tell you the percentage of duty charged.
Hope that helps :)
More reading:
How to Determine Customs Value for Duty
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