Or, quite possibly, you have left out some pertinent details. What precisely did the Apple techs uncover under the hood? More specifics there, please.
IME, both Costco and Apple are very reasonable entities if you are sticking to the warranty policies. You may very well need to contact someone in Costco corporate customer service to help you deal with the Apple side. The whole 'monkey in the middle' type of issues are rarely handled without the two sides communicating.
Get it done, sir(!)
I have a specific issue of Apple warranty support for my iPhone 6. I bought a iPhone 6 in Costco 4 months back, it was identified the phone is restarting automatically every 30 min. I called Apple support team to fix the issue, they could not able to fix it. I visited the Apple retail store, they diagnized and claimed that some of the components are not apple relevant. They wanted to tax on me 300 dollars for a new phone replacement despite Apple warranty. They blaming on Costco would have sold the refurbished item. I mentioned clearly that I got a new sealed phone and is activated in front of me. They are not believing me and are very impatient analyzing the issue. I asked them to contact Costco since Costco is selling apple products. they are not ready to talk to Costco. It is clear that Apple is not taking the responsibility of defective products and is ready support apple warranty. it is clearly mischievous act done by Apple. Please share your thoughts and suggestion on this experience. it is better to expose it to media as well so that goodwill impact will realize the issue of the products and their support