The only test of whether your packaging design works in store is sales. It's not what the design agency tells you. It's not what you think as the founder; you're probably too close to your product to judge how it comes across to outsiders to your brand.
Stage One: Does it catch the consumer's eye?
Look at the amount of choice she has here.
I've just done a project for innocent drinks, interviewing consumers in the aisles.
There is no better final way to check the appeal of a new product before you commit to production runs than to put it on the shelf in the supermarket, and see if it catches shoppers' attention. Do they see it as they come round into the aisle? Do they pause, approach it and pick it up? Does it stand out against its competitors? Will it sell?
In this case, it did. It frequently went into trolleys and off down the aisle towards the check-out. I had to run after people and explain to them that it wasn't quite a finished product, and that, no, they couldn't buy it yet.