Three possiblities.
The seller is overestimating his costs instead of researching them.
The seller is using the Global Shipping Program which adds duty, SALES TAX, and a service fee or around $5 to the seller's shipping cost.
The seller is using an express courier service.
In the first instance, you may have to pay duty and sales tax as well as a $10 service fee on delivery.
In the third, as with GSP, the duty, sales tax and a fat ($25plus) service fee is included in the shipping cost.