You would need to go through Costco Travel - you should be able to see your terms in the email that you received from them. We used Costco to book our August trip, so I just checked for you, and this is what my paperwork states:
Under: Costco Travel Terms and Conditions
REVISIONS AND CANCELLATIONS – Should you need to change or cancel your reservation, or any information provided as part of the reservation, please call Costco Travel as soon as possible. Revisions, cancellations, and refund policies vary by airline, cruise line, hotel, and other suppliers. In all instances, no-shows are nonrefundable.
This is pretty typical for travel agents - I have nothing but good things to say about Costco. We have previously only use Costco Travel for car rentals (by far the best rates/policies around.) We are hoping that this trip is as good.
greg9x is correct - the only Premier resort that Costco has listed is Portofino. We considered the Royal Pacific, until we saw the cost of our package through Costco; the savings more than covered the cost of our flights.