I'd take them back too, the worst they can say is no but at least you will have let them know and have tried. And if they do say no or in addition to returning it, I would contact the manufacturer to let them know. Unfortunately/ fortunately they may send you a coupon/gift receipt for another bag, perhaps it was a bad one or just their flavor but they should know.

The only time I feel badly about a purchase is when I go on a spending spree out of budget. Then I feel slightly guilty if I have to return it, but as long as it is within store policy, it ends up ok in the end. A bad product, no hesitation and no ill feelings about this as all sorts of things can go wrong in a manufacturing process; they need to know about it.

I hope you find a better brew!!