I can't give a price for the taxi, but its probably going to be very time consuming to take one shuttle to Disney and then a taxi to a market.
I would do one of two things:
1. Car service with grocery stop. I've used Happy Limo and been very happy with their service. Mousesavers usually has a coupon for them
2. Use Garden Grocer or the other grocery delivery service (the name I can't remember). GG does charge a delivery fee of I beleive $12 if your order is less than $200, but they deliver right to guest services and guest services keeps your things cold until you call for them.