That's how I send my paintings, rolled up, wrapped in cellophane and taped inside a tube. I recommend gently unrolling it and allowing it to relax for a little while before spraying the back with a fine mist of water (don't drench it, it only needs to be damp, and avoid getting any water on the paint itself). Lay the painting face down on clean towel, on top of a board. Lay another clean towel on top, then weigh the whole thing down with something heavy (I usually use home shopping catalogues - the new Argos one is nice an heavy! lol) Leave it for at least 6 hours and it should be absolutely flat.