First, online and store prices sometimes differ – you will get the lower price. If the online one is lower you can point it out to the cashier and you will get it.
Second, if it is a non-Jewish owned store there is no halachic requirement to go back. It is a Kiddush HaShem to point out the error. As has been discussed here in the past, that seems to be the way most people operate – if you catch the error before leaving the store, go back to be nice. Most folks won’t make a special return trip.