I have the same problem. Normally, when shipping via UPS WorldShip software, I would schedule a pickup for the return. So the customer doesn't have to deal with how to get the packge to the UPS store. I really wish that there were more return options in ShipStation.
I just asked support about this same thing and they said to submit an idea for it so I went here and found the same request for customization of return reasons, restricting non-returnable products, etc. THIS IS FROM 2017! How in 5 years are we not able to implement something so basic as, at least, allowing merchants to set their own return reasons????