Hmm, this is how the product description spells it out:
Adjustable straps allow users to wear like Captain America
Could be interpreted as wearing it on your back, or just being able to hold the shield on your arm. I'm looking at the handle straps and there just might be enough slack there to open it up and put the shield on like a backpack, but it's hard to say for sure.
Wearing it on the arm is a given with any shield because that's how anyone would hold a shield. So wearing it like Capatin America would be like you said, a backpack.