We did not do the grocery stop. We stayed Club Level at RPR and didn't load up the room with snacks. I tipped Bob $15 for the ride to RPR. He had to wait around for our luggage for quite awhile with us. I also had to round up a stray 14yo son who had to eat something before he died of starvation.... While talking to him, he mentioned we were his last trip of the day and he was heading on vacation himself the next day to Indiana. I think I was going to tip $10 originally. (Bob also asked, as I paid him in full for the round trip, to pay gratuities each way as there might be a different driver for the return trip) In our case he knew he wouldn't be taking us back, of course.
I tipped Roger $12 on the way back, only because I didn't think ahead and have a $5 bill on me.
This was the first time we have ever used a car service to WDW or US/IOA and I couldn't have been more pleased. The money felt well spent.
After reading about the $12+/day parking and the hassle of getting a rental car overall (sometimes), I will probably never rent again.
HTH- cortegro