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.
Stage Two: Does she put it in her basket?
Look very closely (click on the shot), she's got her hand on one choice, but she's looking right at another one. So she might well take her hand off that first drink, and replace it with the second one that she's looking at.
Stage Three: Does She Take It to the Till?
Then and only then, has your packaging worked.
So, if you are wise, you'll test design options in store on consumers - for stand-out and communication of key messages - "What is it? Why should I buy it? What will it do for me?", before you commit to signing off a final print run of a design that may not work for consumers. Not to mention a poor rate of sale, which won't do you any favours with supermarket buyers, and will get you de-listed.
Thanks to Innocent Drinks, who gave me the opportunity to really test product stand-out in aisle. It worked for them, so it'll work for you.
Here's one pack design that does catch consumers' eyes in the aisle. And sells.