Cuzco o Cusco o..."Costco"?

I have to admit that I am not the world's best speller.  Add in a second language that frequently differentiates its cognados often by as little as a letter coupled with my sometimes poor attention to detail and, boy, do you have a mess on your hands.  So, imagine my confusion when traveling to what I believed to be Cuzco, Peru, but finding Cusco written as its name on my travel itinerary (created in in America) and then...going on a tour only to be told that technically Cuzco is really Qusquo (originally qusquo wanka or, rather, "rock of the owl"), a Quechua word that is derived from another indigenous language of the area called Aymara.  And, if that weren't enough, when the word came directly out of the mouth of my Quechua guide, my American ear heard "Costco" and thought hmm...when I lived in Mexico they had a Costco (American superstore), could Peru have one too?  Thank goodness the voice of reason stepped in and let me know that it was just a coincidence so that my wandering mind didn't miss anymore of the tour!  Qué tonta estoy…
So, what it is?  Cuzco, Cusco, or Costco?   Find out at: http://gbosma-peru.blogspot.com/2011/08/cuzco-o-cusco-ocostco.html