Buying in bulk as a rule
Buying in bulk can be cost-efficient sometimes, but don’t just have a rule that you always do this. It isn’t efficient if, say, you buy more of something than you can possibly eat before it expires. Then you throw food (and money) in the trashcan. Sometimes it’s better to buy two bell peppers at $1.50 each than six for four dollars if you know you’re only going to make it through three of those before the other three go bad.