You should definitely check with your vet before having your puppy carry any weight at all. If dogs are not supposed to go running for more than a half a mile until they are 18 months old, a backpack doesn't sound like such a bright idea to me.
In any case, for adult dogs, keep in mind that most can only safely carry 1/4 of their body weight in a backpack. Therefore, a 50 pound dog should not carry more than 12.5 lbs, including the backpack itself. You want to make sure the bags are carried over the shoulders and not across the back. Do some more research and make sure to check with the vet first.