Or combine the two;  go into the UV room and apply a new planar map, put on a reference texture (I use a checkerboard) and tweak until it looks flat to the actual shape.

A more advanced technique some Poser content creators have used is to actually deform your mesh until it is flat, map it properly, then reverse the deformation.   Probably wouldn't be great technique on this bag, tho....I'd go with planar-and-tweak.

A last trick for some new model; UV map it when it is still a simple shape, then detail it using only tools that don't corrupt the UV map.  Can be dicey, but when it works is wonderful.