Thanks for the advice, guys! Looks like eBay won't let me revise it to allow international shipping at this point, but I'll definitely start adding it on my smaller items in the future.
I sold a TRS-80 lot that weighed 33lbs, and the estimated shipping to a prospective buyer in Canada was like $200. I kind of gave up on offering to ship to Canada after that, but hopefully it's more sane for smaller items.