So very sorry you had this terrible experience. I also had a terrible experience at Costco, except I made the mistake of laying out the payment for a pair of hearing aids, which I could never get Costco to offer help or support for, when I had issues with it. It also did not live up to its billing of being bluetooth streaming compatible, which I paid extra for.
The thing with hearing aids is, the hardware by itself is never going to be enough to help you with your hearing loss. You are always going to have issues with different hearing environments, with wax gathering in your ears and impeding the sound transmission in your canal, with charging and/or batteries, with changes in your hearing, etc. etc. Your hearing loss is a work in progress, always, and you MUST have access to followup service with your HAs. Costco is simply not equipped to offer it.
Hearing loss is a medical condition with ongoing major impact on your quality of life and on the rest of your health, including your mental health. It’s never going away. Don’t make the mistake of expecting a big box store to offer decent medical treatment. Go to an ENT and an audiologist.
Pay attention to your gut feelings about the poor communication and treatment you got from this retail store salesperson. You are worth more than that. Get with someone who will give you the help you need.