I’m often confused when I see 2 gas stations accross the street from each other, and one is selling it’s gas for 5 cents less, yet people are still at the expensive one. That really should be easy enough on the math, shouldn’t it?
Depends. Is it a station like ARCO which doesn’t take credit cards? Or, is it a station like Racetrac or some non-branded station? Some people don’t like to buy fuel from a non name branded station.
Sometimes the gas station thing is all about traffic patterns.
My personal favorite is at the grocery store my husband and I go to, there is a bin in the middle of some of the aisles - it looks just like the discount bins they have and has a big, brightly colored discount sign on it listing the price of the stuff in the bin. The price listed is almost always precisely the same as the regular price listed on identical items on the shelves.
And yet, every time we go to the store, my dearly beloved announces “Look! It’s on sale!” and starts grabbing them up like he’s worried they’ll disappear. :smack:
I explain, and he always looks slightly embarassed, but it keeps happening all the same. 
Personally, I think that not giving a shit about saving a few cents on each purchase is worth…well, a few cents on each purchase.
Portability.
If you order a small, you only get to take the contents of a small with you when you leave. If you order a large, you can drink all you want at the restaurant, and take the contents of a large with you when you leave.
Same deal with the 20 oz. versus 2 liters. I’m at college. Nobody’s going to haul a two liter from class to class, but tons of people carry 20 oz. For at-home drinking, it’s obviously more practical to buy the bigger bottle, but if you want to take it with you, the convenience may be worth the cost.