Assuming you are going with the Rexton Adore, here is what I see as the First Fit in the Rexton software (the KS9 is not going to be any different though).
The lowest power S (standard) receiver is sufficient for the loss, but if you want to allow for further deterioration in hearing it may be worth going with the M receiver. Something to discuss with the fitter. If you go back and need a more powerful receiver a couple of years from now, I am not sure if Costco will give you one or ask you to pay for it.
The second issue is the fitting type. Signia/Rexton make what they call click sleeves. They come in four sizes and can be open or closed. If you look at the graph below you will see three gain curves for each ear. The top lighter one is the gain for soft sounds. They are amplified the most. The middle is the gain for normal sounds, and the bottom lighter one is the gain for loud sounds. They are amplified the least. This is based on the theory that we lose our sensitivity to soft sounds more than loud sounds. Doing gain this way is what is called compression. In any case for this simulation I used open click sleeves as they tend to be the most comfortable especially for new users. I think your wife will do fine with them for the right ear. The red shaded area needs to be avoid for the gain curve to avoid feedback. It should be OK. The left ear is not so certain that an open sleeve will work. It probably is worth trying, if she wants to go for maximum initial comfort. But, that is an area where you may have to go back and go to closed.
Here is what the same curves would look like using closed click sleeves. They really are not totally closed and have a 1.6 mm vent. There are some advantages to using a more closed fitting. They allow the hearing aids to reject noise better, and allow the directionality features of the aids work better. If you do not want to take the chance of needing to go back for refitting, it would be safer to go with the closed click sleeves. Here is what closed sleeves look like. Note that there is more room between the gain curves and the potential feedback zones. Also note that the left aid in particular is getting close to the maximum output grey zone at higher frequencies. An M receiver would give more margin.
Some other tips:
Ask them to do the Own Voice Processing training and turn it on. It helps with the sound of your own voice, and also enables some other features. It has three settings of strength. Default is medium. Ask to try each.
When you decide on the fitting type ask them to give you a couple of the size smaller and larger than the ones you pick. They are easy to switch at home, and you may want to experiment on which one fits best at home rather than in the rushed appointment time. If you decide on something different in size it may make a small change in the REM, so that should be redone at the next appointment.
You should also go prepared to tell them which programs you want turned on. Here is a link to the similar KS8 Kirkland model quick start guide which gives the main options. There are sub options to the music program depending on what you want to do with it.
KS8 Quick Start Guide
If you are going to use a smart phone bring it with you so they can help you pair it the first time. These aids will work better with an iPhone, but they can pair with an Android. You should download the suitable app ahead of time and go there with it installed.
The buttons on the aids can be programmed to do different things. I find it handy to have a long hold on any button to mute the aids. There are times in a noisy environment you just want to shut them off without taking them out of your ears.
Bottom line, if you want to minimize the number of fitting trips, go prepared with a list of what you want done.