Perhaps I am the last person on the forum to discover this, but Costco members get a 15% discount on tours that they purchase through Viator by using this portal https://www.costcotravel.com/specialty/viator
I'm not interested in entering into a debate about 3rd party tour consolidators, simply stating that if you are going to use Viator and are a Costco member, this savings is available to you.
If you have a Chase card, they currently have a July offer of 8% cash back on purchases from Viator. You can combine them and come out with savings of just under 22% which is pretty nice.