It’s not duty.
EBay recently introduced a Global Shipping Program which is run by Pitney Bowes. The extra money is the fee that PB charges you for slower delivery and brokerage.
It is best if you are willing to pay over $20 for an item from the USA to ask specifically of the seller before bidding if he uses the GSP and let him know, if he does, that you will not be bidding due to the increased cost and slower delivery with this “service”. (The seller gets a tiny break on his shipping fee for using it, but the effect on the customer is not explained.)