As I want my car boot trolley concept to fold into the car, Mike suggested looking at the way stretchers fold into the ambulance.
Ambulance stretchers work by two sets of wheels. One set on the legs of the wheels and one on the bottom of the stretcher. Where the legs of the stretcher hit the entrance of the boot the hinge folds them down and uses the wheels on the bottom of the stretcher to wheel it into the boot.
By using this method or one similar there will be no need to add a ramp or other accessory into the car. It will illuminate the need for additional stress of getting the shopping in and out of the car. The trolley can be pushed straight in and pulled out lowing the legs and turning it back into a regular trolley.