Resisting the urge for more

It even happens to me - there I am standing at the register in the pet food store buying my 2 bags of cat food when the clerk says " you know, if you buy 3 bags you get a 4th bag FREE". What am I suppose to do now? I buy 2 bags at a time because that's exactly what I have space for. Two bags, that sit neatly on the shelf, side by side. Three bags, yes, I could probably find room somewhere for a 3rd bag, but the 4th bag - I'd have to buy a bigger condo and that isn't going to happen. So I buy my two bags, but I can't stop thinking about the FREE bag of cat food I left behind.