Barking at Shopping Bags

Theres just one thing we cant understand with our Lab..If she sees anyone on walks with shopping bags she growls and barks .Never bares her teeth or snarls ,its more a fearful thing.
No idea where it came from.We got her past her fear of balloons and wheelie bins when she was a pup but now 4 years old .
Our regular postman is a dog lover and when we see him out an about she will suddenly stop and bark quite fiercely. I told him it was his post bag so he took it off and approached her and she welcomed him and all the fuss!
She doesnt mind either of us with a bag...so any ideas how to deal with this please ?

I too am in a similar situation, only mine doesn't bark at the bag (not particularly shopping bags, more messenger or hold all type bags), she'd like to attack it or the person it is attached to! I also think its a fear issue as when the bag is either out of site or off the person, she will quite happily go to the person to have a lick or a play. Why? And what can be done to help apart from trying to get her used to people with bags

get people with bags to walk by and throw chicken at her