The actual cost for this is $20.03 (small package tracked) from Canada to the US. The calculater charged $17.01.
USD or CDN? Are you being confused by the exchange rate? Today the xe.com site is showing a mid-market rate of 87 US cents to the loonie. But that rate only applies to huge multi million dollar transactions between banks. Your own bank would charge you more.
If you sell in USD this is an easy mistake to make. And most of us do sell in USD.
Are you using metric measurements? Are you making an allowance for the size and weight of packaging? Did you know you can add a handling /packaging fee to your Calculated rate which is not broken out to the buyer?