Have you tried using Calculated Shipping? Many buyers will just pass a listing by if they cannot get the cost of shipping immediately.
Two tips. Use metric measurements. eBay's default is Imperial which Canada Post has not used since 1974 and using it leaves you open to errors.
And double check when you upload a listing that the US shipping in particular shows up. There is a tiny difference between Canada Post standards and USPS standards which eBay does not warn about. If the US shipping does not show up, hit Revise, go back and add a centimeter to your Canadian dimensions and try again. Don't worry , this does not change the Canadian dimensions enough to increase the rate your customer will see.