The Mears shuttle will not do a grocery stop. You may be on a shuttle or bus with guests from other resorts, so be prepared that the shuttle may take awhile if you are not the first guests to be let off.
One of the problems with booking packages is some of the perks really aren't worth it. For example, the Mears roundtrip service costs about $25 per person. For roughly $80 roundtrip total, you can have a private towncar pick you up at the airport and drive you directly to your resort. Most of the companies will even include a free grocery stop if you want it. After doing a package in 2001 and using Mears, we did the towncar service in 2002 and will never go back to Mears. We didn't have a bad experience with Mears, but the towncar was much nicer. With only two of us, Mears is cheaper, but we like the grocery stop and not having to leave our resort as early to get picked up for the airport. Also, with a family of 3 or more, it is actually cheaper to do a towncar service. Hope this helps!