This happened to my husband and me once at a store. We purchased several items and when we got home, we realized that they didn't charge us for an expensive piece of software. We called the store because we lived about thirty miles away on an island. We asked if we could pay for it over the phone. They said we would have to bring the item back in and pay for it in person. We asked if they would compensate us (with a discount) for our time and gas to come back to pay for it since we did not need to go off island again for quite awhile. (It was their clerk's error, not ours.) They said they could not, but that they expected us to pay for it. My husband, who was getting ready to deploy for three months, told them he would come back and pay for it when he got back from his deployment. Three and a half months later, we drove back to the store and the clerks just looked at us funny and said not to worry about it because they didn't have it in their system anymore. Aargh! We tried.