Where is the buyer located? What did you sell? When you say it weights "around 1kg" did you actually weigh the whole thing with shipping box to see if it's under 1kg or over 1kg? Is the height + weight + width under 90cm? The shipping cost is very different depending on the actual weight and size of the box.
For best seller protection you need to ship it with tracked packet, which is $$$. Or you could ship with $77 airmail at your own risk as there is no tracking. Never ship via surface mail, especially when you're a new seller because that's just asking for trouble. It takes too long to reach your buyer and your buyer could file an item-not-received claim after a month and you'd be out both the item and shipping cost.
@canehdia wrote:
At this point should I message the buyer to let them know that I mistakenly left international shipping open and that the shipping will end up costing them a lot, and that I can cancel the order if they want?
International shipping is a bit complicated especially for new sellers, so if you don't want to figure it out (understandably), I'd just reply with "Sorry [buyer], we actually don't ship outside Canada and this listing was set up incorrectly. Sorry about the inconvenience but we would have to cancel this sale. Thank you for your kind understanding." and leave it at that. Don't bother saying any more than that (don't tell the buyer you are cancelling the sale because international shipping is $$). Then you can cancel the sale by choosing "problem with buyer's address" so that you don't end up with a defect.
After sending that message, make sure you adjust all of your listings to indicate that you only ship to Canada. I'd suggest that you ship to US as well... US shipping is not as confusing, not too costly and it's a much bigger market than Canada.