Unfortunately my experience was just the opposite, I was denied years ago. I suppose it depends on who you talk to in the store. Either way it’s good to see you received the price correction.
I don't know the details of your experience, but it does NOT depend upon "who you talk to in the store," AFAIK it's a Costco policy. If someone says "no we don't have a price adjustment policy," ask for the operating manager. (I read on Reddit about new associates at busy stores that get things like this wrong, and can get their butts in a sling, or be fired.)
The policy can be confirmed by calling their customer service number. Here's the policy for online orders:
https://customerservice.costco.com/app/answers/detail/a_id/628/kw/price check
Of course, if you get someone who is ignorant of the policy, and the same product is on sale at the store at an even lower price, you could bring in the old receipt (higher price), buy the TV (new lower price), wheel it around to Customer Returns, and return it with the old receipt. NO they do not say "you just bought this one you can't return it." ... If you've figured out "oh, this is why they do price adjustments," yep it's a lot less hassle than wheeling big TVs around teh store.
BTW they'll also do adjustments for things you just bought within 30 days and suddenly there's a rebate, too. Some years, the coupon/rebate gods know what I'm buying because I get price adjustments several times when there's a sale (or like this year when there was a rare "buy one, get one" offer a week after I bought one).
picked up a 55C8 at Costco zip code 95340 for $999. They still had 4 left. 65C8 is $1799 and they had a stack of those.
I don't think you can search online for the 55" but perhaps you can call your local Costco to see if they have any left. Product number is 9550018.
9550018 isn't online. Considering the 55E8 and 55C9 are roughly $1600 (Costco pricing) online, that must be a great deal. Congrats man!