9 mo is a very common age for GSD to start " feeling their oats" their hormones are kicking in, their natural protective instinct is kicking in but their mind is not mature enough to handle it. So they bark and hackle.
My suggestion would be to have a basket of treats at the front if the door with a sign saying " puppy in training, please take a treat and help us teach him people are great!" If he won't take the treat, tell customers to drop it on the floor so he can get it. Keep socializing him. But he is probably starting to think the store is HIS to protect.
Most well bred good nerves dogs outgrow this very annoying behavior.
My 9 mo old does this in the neighborhood. But nowhere else. Usually I refocus him and ask the person if my puppy can say hello. Then we approach.
I would not be overly concerned if this is the only place he displays this behavior, in regards to therapy work.
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