I really can't imagine why they would say it only includes expensive cars. I'd be pushing that as in 'tell me more, do you think I'm an idiot?' Push for a reason, if they can make the statement truthfully they will know why, or they are simply lying to you.
I'd be trying to pull some of that fuel and checking for water content if you run legally mandated ethanol in the fuel. Wouldn't the first time ethanol got left in the open and water condensed in it to make plenty of misery.
Or maybe the fuel got out of whack as to solvent content or similar.
Getting a letter and the fuel system screwing up are too coincidental to me. But then I've watched here in Texas while occasionally they mismix fuel and the blame goes everywhere with finger pointing galore. Last time it was Walmart fuel, Walmart was so quick to distance itself from its' supplier it was breathtaking. Talk about throwing someone under the bus.
I'd be asking them if the car knows whether or not it's a Mercedes before it rolls over and dies............................LOL. (not at you of course)
I'd also give THEM a present, no more fuel purchases at Costco. Fuel savings are useless if they are damaging your car.