Buyer wants me to pay for import fees

So I have a buyer from Canada who ordered something from me for about $21.
They contacted me the other day saying that upon delivery they were charged
$12 for import fees and they paid it to receive the package.

Now the buyer is telling me that they didn't know about any import fees before
buying the product and since I also didn't mention anything about import fees in the listing
they think it's my fault and wants me to pay them back the $12 they paid to the postal man.

My question is, as…

Buyer wants me to pay for import fees