[TokyoSeven]

First time, you will receive a letter that states that your item is classified as prohibited for importation and that the item has been confiscated by the CBSA. Information and a time line will be given to you if you wish to dispute their decision.
After this your name or address maybe added to a list where they will make sure to check your incoming out of country parcels.

Second time, Same letter as the first, your name or address will definitely be added to some form of watch list. You may receive a letter siting the the laws of importation and a statement asking you to not do it anymore, possibly a fine, possibly jail time.

For the most part the CBSA considers an airsoft gun to be a prohibited device and attempting importation can even land you in jail for minimum of one year if you get caught according to the criminal code.

When crossing the border Failure to declare restricted and prohibited goods may result in penalties of up to $400, permanent seizure of goods and in some cases, criminal prosecution. A record of infractions is kept in the CBSA computer system. If you have an infraction record, you may have issues crossing the border in the future and may have to undergo more detailed examinations.