I'm going to use your dog food as an example, so this doesn't get too confusing.
The next time you go to the store, get your dog food, and have the cashier ring up two of them, even though you only have one. Explain to the cashier what happened, and that you need to pay for the one that was forgotten. If there is a question, still, from the cashier, talk to a manager then. Accidents happen.
I've had this happen a few times, as a csahier and while it is a little confusing, at least the item was paid for.