If your purchase merchandise in Alaska, as you describe, there should be no duties applicable. Note I use the word :"should". Merchandise purchased in Canada exceeding your $400 per person limit couldbe liuble for custom fees.
I recommend that you get a free booklet from the U.S. Custom Service entitled "Know Before You Go" Mail:Custom Department of Treasury, U.S. Custom Service, Washinton DC 20229 or you can go on the WEB for a print out of Know Before You Go. You can download it and it prints out 22 pages,
http://www.customs.ustreas.gov/travel/know.htm
Have a good trip.We will be on the HAL Prinsendam to Alasaka from August 8toAugust 23. Auuist 23
Auf Weidersehn