You have options.
First, weigh and measure your item, in its packaging, and work out how much it will actually cost to ship to the locations of your bidders.
canadapost.ca -- and look at the Business prices, which are more flexible than the Personal ones.
If any of those prices are too high to cover shipping, go to the listing and CANCEL those bids. It may be that shipping to Brazil is prohibitive but to Norway is fine.
Block the bidders whose bids you cancel. CYA.
BTW, all that calculated shipping requires is that you enter the dimensions and weight of the parcel on the Sell Your Item form. It is important to check the Options button and choose metric measurements from the dropdown. Canada Post went metric in 1974 and any other measurements will be approximate. You need accurate.
AND - only look at AIR MAIL shipping for overseas. Do NOT even think about Surface. Just don't.