A stained glass panel is heavy leaded glass held together by soft came that sags under its own weight. The rule that saves it is simple: crate it upright and never, ever lay it flat.
Pro tip: For valuable or large panels, professional art crating is worth the cost; the crate is the difference between arrival and disaster.
Watch out: Never lay a stained glass panel flat; the soft lead came sags and the glass cracks under its own unsupported weight.