I would suggest just returning the pack, and waiting until your dog is full grown to buy another. There is no reason to stress a growing dog's joints by adding extra weight to them. It's a good idea to wait until they are fully grown before using a backpack.
Jubal/Tesla (Beauceron) 3/14/14
Bianca HIC CGC TT (GSD) 4/24/04-10/23/12