I have often posted about this. Here is a recap:
Up to 18 cards - penny sleeves or 3 pockets cut from a 9-pocket sheet, in between two pieces of cereal boxboard in a business PWE - standard international mail rates (Canada $1.20). I don't put a return address so that an irate postal worker cannot return to sender if they think there is not sufficient postage.
20-100 cards - in a bubble mailer, two to four stacks in between cereal boxboard in a bubble mailer to accomodate size. Cost by weight - up to 200 grams - $3 - $5.15. Once you cross the 200 grams, cost increases substantially.
For the two methods above, DO NOT put a customs declaration on it or it will have to go out as a package and yes, $10 for the States to ship to Canada.
Over 100 cards, think about making the trade for 250-300 cards - in a small collector's box to fit cards - $16 or so. Customs declaration is necessary. Possibly think about shipping in two bubble mailers described above.
Chuck hit on the regs - no more than 3/4" thick and must be "paper" or flexible. Find the postie that agrees with this interpretation of the rules or package and post yourself through Paypal. My PWE never go through the postal wicket. I stick a stamp on it and drop it in the mail box. Other than one to Saskatchewan, all have reached their destination. No one has told me that there was postage due. I do not believe that USPS will collect postage due for Canada and vice versa.
AVOID THE POSTIE WHO INSISTS EVERYTHING IS A PACKAGE AND NEEDS A CUSTOMS DECLARATION!!!!!
Bruno
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Check my Profile page to see my 2023 Goals and my Lists of sets near completion (5 cards or less) or sets getting close (less than 100 cards missing and 75% complete).
https://www.tcdb.com/Forum.cfm/Page/B/ID/0/?MODE=VIEW&ThreadID=25745&C=0