Actually, your prospective US buyer will have seen nothing about free shipping on the item because you didn't set up your listing with a US shipping method and rate. Instead, they'll have seen this:
and this:
. . . so I think you're perfectly within your rights to give them a shipping quote to their location and send them an invoice with the US shipping charge on it should they commit to purchasing the item.
In case you're wondering, I changed my shipping location from my postal code to the USA with a US ZIP code, so that's how I was able to get those messages to come up on your listing.
It's interesting how this prospective buyer found your listing, actually. as it shouldn't have come up for them in a search if they were browsing the dot-com site. Perhaps they've found they can find the occasional bargoon if they search directly on the Canadian eBay site instead of the American one.
