I have not used the specific products you are talking about, but I have a few pieces of information that may help.
1. Not long ago, Congress passed a law authorizing the sale of over-the-counter hearing aids. These are devices that are more sophisticated than the simple devices you are talking about, but less sophisticated than the ones audiologists dispense, and the consumer would do more of their own adjustments, possibly with the help of a smartphone app. As a brand-new type of device, they have not yet hit the market but will be coming soon. So one possibility would be to hold off a bit and see if those types of devices work for your wife.
2. HOWEVER....another possibility you might consider is to go ahead with the expensive, audiologist-fitted devices for the first round. (First, you say? Yup, hearing aids have to be replaced every five years or so.) Your wife will learn so much from the process that is almost impossible to research--about what she can expect from the technology, likes and dislikes, the kind of technology that works best for her, options available. Then next time, it would be much easier to go with a mail-order purchase or an OTC device, or to decide that audiologist fittings continue to be the right choice for her.
3. If you have insurance, check to see if they participate in any hearing aid discount programs. Mine doesn't have hearing aid coverage, but I was able to use a discount program called Amplifon. The audiologist said it saved about $1000 on my device (I only wear one)--who knows on that, but I also got a ton of batteries included, extended warranties, and much better terms on trial periods and exchanges. There were actually a couple of different programs and I ended up picking sort of randomly because I was so overwhelmed, but it worked out well for me.
4. Keep in mind that hearing aids are more like a prosthetic limb than glasses. You don't just stick them in your ear and go...you have to learn how to use them and your brain has to relearn how to hear with them. The first few days can really suck. It probably took a month for me to be fairly comfortable, and around six months before mine worked perfectly for me. A LOT of people, especially ones who aren't being coached by an audiologist, try them out for a few days, get frustrated, and throw the aids in a drawer claiming they didn't work right. Talk about a waste of money...
5. Hearing aids dispensed by an audiologist have a minimum of a 30-day return policy in my state, and I actually had 60 days due to the discount program I used. I actually did try out one other device and returned it before I settled on the current version. So you can try but not buy. (There may be a restocking fee of a couple hundred dollars to compensate the audiologist for his or her time in the fitting. That was waived under my program, but I know it was part of the standard contract).
6. The Costco aids, from what I hear, are perfectly fine but you're right, you need to be convenient to a store for adjustments. I didn't bother with them because they didn't carry the kind of hardware I wanted to try initially, but they can be a great option if convenient.
7. If your wife is a musician or cares a lot about music, skip over the advice above and just spring for the fancy high-tech aids. They really do make a difference to people who are trained to notice. Tell the audiologist she needs the "music" setting.
I also just want to say...I resisted hearing aids for several years, partly because of the cost, and I so regret it now. Hearing technology has changed my life and so reduced the social isolation that was developing, and problems I was having at work because I had trouble communicating. (It also gives you superpowers like turning off your hearing when your annoying cousin starts yammering about politics...)
It's surprisingly hard to find useful information about living with partial hearing loss--a lot of the resources out there are for fully deafened people, or the parents of deaf children--but I did like this book:
https://www.amazon.com/gp/product/0761187227. Good luck!