Costco have universal high standards that each Costco is obligated to follow. Their testing is very complete and when fitting aids they do Real Ear Measurement, which many private shops do not do. That said you can run into rogue fitters who are not following the standards. It is best to go in and talk to the people at the one you are considering. If there is more than one choice and you don’t get a good feeling about the first one, try another. A couple of years ago Consumer Reports did a survey of about 17,000 readers who had hearing aids, and Costco rated #1 for a retailer. Their Kirkland Signature aids rated #1 for a brand. At the time the survey was done, the Kirkland models were made by Rexton/Signia. The #1 brand was Signia.
Costco really only carries premium level hearing aids. Their aids get different names than you will see in other non Costco shops, but they generally line up to the premium level aids elsewhere. For example the Rexton Adore Li is essentially the Rexton Emerald 80, or Signia 7Nx. They typically only carry demo units at each outlet and after you pay for them, they will order you a set. They should have all the units shown at the Costco Hearing Aid Website. If you keep following links down, you can get details on most of them.
The current Costco Kirkland Signature branded KS9 aids are essentially Phonak Marvel M90 312 aids. At $1500 a pair it is really hard to ignore that choice. At private clinics the Marvel M90 may cost as much as three times that!
If you go to a Costco to get a hearing test and try out some demo aids, it may be better to do it in two appointments. Their initial audiogram testing takes a long time. You may want to use that appointment to all the testing done, and gather information on the choices and prices, but then come back for a second appointment to try the one or two you are interested in. For sure insist they do a Real Ear Measurement (REM) adjustment on the aids you are going to test. Most will allow you to go for a walk around the store with the demo aids to try them. But if you want to know what they are really going to sound like, you need the REM adjustment made.