Keep them on the frame and ship them that way. You and your customer will be much happier. Someone's going to have to put it back together, you know. Wrap it in paper, cover with foamcore (styrofoam bracing is a good idea too,) and bubblewrap the heck out of it. If you build lightweight wood cases for them, all the better. Charge extra.
If it's a gigantic painting and too large to ship on the frame, then rolling it up is the necessary option. If you must do this, cover the work in paper, and roll it carefully paint side out over a large sturdy cylinder of some sort. Restretching a painted canvas is no fun.
dbclemons is right, shipping it rolled will do you no favours due to the receipient having to have it restretched. They are unlikely to know how, and will have to pay for it on top of the framing.
The number one problem with shipping paintings on canvas is the fact that it is very easy to have something stacked/dropped onto the stretched area, easily tearing through it. Bear this in mind when packaging, and pad the back of the canvas out thoroughly between the stretchers to stop this happening.
Depending on the size of the works involved, she ought to look into mounting the canvas on to board (I mount canvas to 6mm mdf). This is what I do for sizes up to 16" x 20". The packaging is much easier, due to the painting being far more sturdy, and also much cheaper due to less packaging materials.