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No. Postage is valid only in the country that issued it. A US stamp is payment to the United States Postal Service, so Canada Post will not accept it on a letter dropped in a Canadian mailbox. If you are mailing from Canada, use Canadian postage. If you are mailing from the United States, use US postage. The destination does not change which country's stamps you need.
A stamp is not money. It is a prepaid voucher for one specific carrier, and each national post office only honours its own. When you drop a letter in a Canada Post box, Canada Post is the one hauling and sorting it, so it needs Canadian postage to be paid. A US stamp on that envelope pays nobody who is doing the work. Sorting machines read the indicia and reject mismatched or unrecognized postage, so the letter usually comes back to you, sometimes after a long detour.
The most likely outcome is that the letter is returned to the sender for insufficient postage, if a return address is visible. Without one, it can sit in a dead-letter stream or eventually be delivered postage-due, meaning the recipient pays a fee to receive it. Either way it costs you time and, quite possibly, someone's goodwill.
| Where you mail it | Where it goes | Which stamps to use |
|---|---|---|
| Canada | Anywhere in Canada | Canadian domestic stamps |
| Canada | United States | Canadian stamps at the US rate (Canada Post sells stamps denominated for US mail) |
| Canada | Overseas | Canadian stamps at the international rate |
| United States | Canada | US stamps at the international rate |
The pattern is simple: the country where you post the letter is the country whose stamps you buy. A US Forever stamp covers US domestic postage only; it does not become international postage just because the envelope is addressed to Toronto.
If you have not mailed it yet, do not try to "top up" with Canadian stamps alongside the US one. Postal clerks will tell you the foreign stamp counts for nothing, so you would be paying twice for no reason. The clean fix is to start a fresh envelope with correct postage. The US stamp is not wasted: stamps do not expire, so save it for the next letter you actually post in the United States.
You cannot send your Canadian correspondent a US stamp for their reply, for the same reason. The official tool for this is the International Reply Coupon, a voucher sold at post offices under a Universal Postal Union treaty. Your correspondent exchanges the coupon at their own post office for enough postage to mail a letter back to you. It is old-fashioned, but it is the only prepaid reply mechanism that works across borders.
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