I have never seen the need to put a pack on the dog, myself. But if you do, check and test the design of the pack VERY carefully. The middle of a dog's back is a weak spot, so the pack should ride over the shoulders and not slide back.

Mail order isn't really suitable for dog pack shopping. Go to a good pet store with the dog and actually try them on. Add some weight and see how rides and if it shifts. Check for friction spots from the straps when loaded.

Continually check the pack when in use.

Try to find one with a top-mounted quick-release in case you meet a predator, as you'll need to get that pack off instantly.

Sue