Costco customers trend mid-upper income and are often small business owners. So they'd want something with frugal mileage, but a little style and an interior that feels a bit more exclusive.
There should only be two models, both crossovers. 'Base' models are mid-range.
Mazda's crossover's are perfect for this. Skyactiv engines, sporting pretensions and budget-Audi interiors create a premium, European air. Kirkland Signature is a well-regarded brand, so why not just call the car brand 'KS,' give it a stylized logo and the model's are named after displacement? Luxury models get an 'L' so it's KSL.
Mazda CX-3: KS 20; KSL 20
Mazda CX-5: KS 25, KSL 25