I'm not sure I get what you mean. If I go to the store with a list of the usual items I purchased and some of those items are on sale, I end up spending less money for the exact same items.
Anyhow, sorry for being confusing. It's possible this example isn't all that common in general. I just bump into it all the time.
We have some Belgian waffles that cost 18 NOK a piece. They always have an offer that gives you 2 for 20 NOK.
I never eat more than one of these and they end up spoiling as they don't last very long, and even if it seems that I'm "saving" 16 NOK on the purchase I'm actually spending 2 more than I otherwise would.
Often the "deals" are "buy 3, get one free", where the per unit price might be a bit less than a single unit, but if you only went in for 1, but ended up buy 4, you've paid more than you might have otherwise. Sometimes it's a good deal, and you get more value, sometimes the items expire before you end up using them, etc.