Grab is driver's own private car, so of course they will be more careful not to let ppl eat. Some people eat like nobody's business and wipe here and there making the car dirty, although some will be more careful and make sure food doesn't spill. Try do that in a taxi and you see what happen.
Any aditional fee like tol, parking, entrance fee etc, passenger have to bear. That's the price for cheaper transport.

Still you have to ask permission of the car owner. If those not careful eater, cleaning the car seat could cost a lot.