I think you're going too far Dennis, infact it sounds to me like you're taking the piss a bit - which is out of order.
So long as somebody is on hand to help them shop (by pushing the trolley, putting stuff in the trolley, then packing and loading/unloading car) they should be okay just trundling along behind in their mobility scooter telling the assistant what they want. Most scooters have a basket on the front into which they can put a couple of bags of donuts, pastries, crisps, fried chicken etc... to tide them over.
I really dont think it is much to ask.