Generally when you think of Cargo carriers you think of a box or bag on the top of a SUV or minivan. These top type carriers add a lot of drag (read hurts gas mileage). Plus these top carriers are a pain to access. If grandmother's home is very far away, then your going to be more frustrated.
Thankfully, I found a different solution. A large box that kind of looks like an old steamer trunk that mounts on my receiver hitch. The best feature is that the box swings away allowing full access to the back lift-gate, window, or heaven forbid spare tire.
The box comes in black or white; both look good on the back of your vehicle. For safety there are brake lights built into the unit (connecting via trailer wiring socket) and a license place holder.
The max package includes several useful accessories that greatly improved my product satisfaction. 1) steel wheels allow easy storage 2) anti-wobble hitch coupling tightener so it was quiet 3) locking hitch pin
I used this on a 2500 mile vacation last year and was amazed at easy it was to access my cargo, securely lock the box, and keep everything weather-tight. We went through the hardest rainstorm I have ever driven through and no leaks!!!
At $3.50/gal gas why would consider a top mount box that would hurt your mileage--use a hitch mount box like this great one!