I think it really depends on what you're getting. For groceries, I generally only buy the half off stuff. Parts are interesting because unless you know exactly what to get, you might end up running in circles. Both vehicles I have are 1998 engines, yet one is a 2000 and one is 2003. It was fun to figure out what serpentine belt I needed.
Another example, manufacturer sells a part for a steam table to regulate the temperature. It was $120 for one part. Alright so I buy one for a customer. Then I did some research. Bought 8 from the UK for the price of one shipped to the US.
Hehe yeah, I'm not sure how to explain that to a second grader.