My sister is a doctor of audiology and both my parents have hearing aids. My mom has severe hearing loss and has a cochlear implant plus hearing aids. My dad just has hearing aids and has moderate loss.
There are two things to consider when choosing Costco. (1)The quality of the hearing test and fitting and (2) the actual hearing aid. For many people, Costco's level of quality will meet their needs and be "good enough," but if you've never had hearing aids or had hearing loss evaluated before, that can be difficult to assess.
Costco has hearing aid fitters that lack the same level of training as a PhD audiologist. That said, some hearing aid fitters have been in the field longer than there has been an audiology doctorate and they do fantastic work. Basically, the level of training is so brief that it's very hit-or-miss. With Costco's great benefits and how well they treat their employees, I'd expect that they have some the better fitters out there.
I'm not 100% positive but I can't see how Costco's testing could be as extensive as an audiologist's. There's no way they could give away a 30-60 min long evaluation involving the expensive equipment, such as a soundproof booth, that my sister has access to. With more limited equipment and a more limited test, they'll be able to identify most mainstream hearing loss but you're not going to get the same level of detailed hearing evaluation.
The last piece of the puzzle is the fit, which can make or break the hearing aid. If the hearing aid doesn't fit well, irritates the ear, or falls out frequently, it won't be worn. If it doesn't fit well initially, you have to return (possibly repeatedly) for additional ear moldings and fittings. My mom has been through several audiologists and this part has been highly dependent on the person making the mold and fitting it. Some seemed to never get it right while others have been able to make it work well.
My sister says that Costco has some good hearing aids and some that are so-so. The good ones are more expensive and are basically older models of what she fits and sells. The so-so ones are better than nothing, for sure, but there is always the issue of actually getting someone to wear them. Hearing aids are annoying to have in your ears and if they don't work well, many times the person with hearing loss will just skip wearing them,
I know you said that she doesn't need bluetooth or anything fancy, but most of the higher quality hearing aids will have those features. It's pretty amazing to see my mom pull out her phone and adjust her hearing aid on-the-fly to correct for background noise or to compensate for a voice she has trouble understanding. She can also receive phone calls directly in her hearing aids which has been huge as she had to put everyone on speaker before (and hold it up to her ear).
I'd look for an audiologist in your immediate area for a hearing evaluation and discuss hearing aid options. (Good audiologists just want their patients to be able to hear and I know my sister has sent patients to Costco when the aids she offered were out of their price range.) Once that's done, you can determine if Costco is a good fit for your mom. You'll save some money with them, for sure, but if it's a 2 hour trek, I don't know that it'd be worth it.
Hope this helps!
Sarah