The Costco Visa offers a 3% rebate on all travel purchases made with that card; it's completely independent of who you are booking through, because the payment, in the end, is made directly to
DCL. I've booked cruises, hotels and airfare all on my Costco Visa and earned the 3% rebate without using Costco Travel at all. The cruises and some of my Disney hotel reservations were made through my
travel agent. The rest I booked myself.
The 2% Costco Executive reward is for executive members who use Costco Travel to book. I don't believe you have to use the Costco Visa to pay for it, but I'm not sure why you wouldn't. The Costco Visa has some of the best non-exclusive travel rewards out there. (Some cards may offer higher rewards, but only on travel booked through their portal.)