Ironically, my main reasoning for getting the Amex was that Costco doesn't take Visa/Mastercard/Discover. Non-Amex is far less convenient to me than Amex.
My trick for using my credit card of choice at Costco is buying Costco gift cards on their website (which, unlike the brick-and-mortar locations, accepts all major credit cards) and then using the gift cards as my form of payment at the retail location.
I was about to shout "Brilliant, absolutely brilliant!" until some other part of my brain said "But what about the rebates?"
I'd love to try this because at the moment I despise AMEX. I scheduled an electronic payment that did not go through (while I was on vacation, natch), so I got a nastygram raising my (theoretical) interest rate to 29+%, threatening my credit, blah, blah, blah. I think it's rather hilarious in an irritating sort of way, because I'm FI, don't give a shit about my credit score, and absent snafus, always pay in full. I've also had an AMEX for over thirty years, which apparently matters not a whit. I quickly made the payment and have been using the card as little as possible ever since. If there's a workaround, I'm all ears. I'm guessing that gift cards don't earn rebates, but please, please tell me there's a way to have my cake and eat it too.
What pisses me off about the whole debacle is this: What about the poor schlub who misses a payment because he/she is struggling to get by? One missed payment and you're forking out almost 30%? What a bunch of crooks.