1. Actually why not give your customers an option ? Let your customers have an option on what kind of after-sales service and price them accordingly like pickup service, onsite or carry-in. Even Dell does that , Illegear does that. I can see that you might have a high overhead cost, but utilizing your resources efficiently is key to business success. You see, not everyone needs a full 3 year warranty for GPU, some rich buggers change GPU every year, I know that. So what is the use giving these group of people full length 3 years warranty if they will not utilize them. They might only need the cheapest price possible . I am aware that you cover warranty services for the entire duration stated on the box of the product unlike some shops that will charge processing fees after 1 year post purchase. Let me give you an example, last time when I buy mobile phones, the shop will offer me either AP set or SME set if I buy Samsung. Of course, warranty terms will be different, but at least I am given a choice.

2. Cable management is a given these days if you buy a full PC set these days even in lowyat plaza. So anyhow you have to match your competitors out there. Stress test doesn't need to be done that long, as long as it is stable enough for an hour for stock settings, that is about it. If it fails stress test at stock speeds, the component is faulty. Then it is your responsibility to deal with your supplier. Besides, you dont have to sit down right in front of the rig to stare at the screen for 1 hour doing nothing. Just leave it running and do other stuffs.
OC can be charged as additional service because I know that it is very time consuming. However, not everybody wants to OC their rig. Some demand absolute stability for production work, so they do not want to OC. These days, many people buy gaming rigs for video editing production work or Youtube streamers because of GPU acceleration support on software. Besides, stuffs like RAM can get unstable when OC if you hit certain temperature. You do not know your customer's premise ambient temp to guarantee stability, you do stress test at your own premise. Of course, merely activating XMP/DOCP on RAM is not consider as OC service.

3. For old PC which are out of warranty, you do FOC checking and stuffs but I'm sure you will try to convince your customers to upgrade if possible. That is sales and marketing for you. You put a car wash example, so I will put another car example. During Hari Raya season, alot of companies like Perodua and Proton will advertise " Free inspection" services. Yea, the inspection service is free, but don't you think they will try to sell you some stuffs ? If you hard sell people, customer angry and run away. So you have to know the limits very well, cost of free services can be offset with sales too.

I do not want to bash anyone. That is just my opinion as a consumer. Consumers always want more choices and listen to customer feedback.