If it's relatively "affordable" for your parents and they don't need handholding, Costco will likely be the cheapest. But you won't get service. But unlike booking directly with Viking, you might get a cash card from Costco. When I have used costco for booking cruises (I do all my own planning) they had the same perks available as I would get had I booked directly with the cruiseline. That might not always be the case. For more complicated, expensive trip - and for people who want/need help and some handholding - use a travel agent. You can see who many of us here have used (we use Pavlus - it was great, then we had a dud of an agent when our original retired, we just switched this year and have another good agent) They should be able to offer you the same perks as buying direct and potentially other perks as well - just depends on specifics. (no experience with amex travel, sorry)

Mainly think about what is most important for your parents (and you, if they are expecting you to do any/all of the planning for them) - the price, the service...You can always pick a few ways to get quotes right now and see how each responds.