I don't often shop at Costco, but I do buy most of my gas there, because it's got the best prices for gas by far.  I always use my post-cruise Costco Cash Card to pay at the pump.  That 8.5% of the base cruise cost they give back is the best deal for me.  Combined with my executive membership 2% reward, and my Costco Visa card 3% reward, that's 13.5% total of my base cruise cost back to me.  That's unbeatable savings on these expensive cruises, and for that I'll put up with one or two long hold times.  When I can I book directly via 
DCL first & get all the details nailed down, and then transfer to Costco afterwards.  That cuts down on the need to call.