It is an OBB. I booked thru Costco yesterday Aug 27 Alaska cat 5C. I called
DCL later in the day myself to get quotes on other categories and found that for the same price as the 5C I can get a 7A and a 11B. So now I’m wondering if I can change the original booking without losing the OBB benefits. Of course the phone lines are ridiculous today so I’m not even trying to get through to Costco.