It's not an "overblown" price. You're paying for the customer service you get as well as some convenience.
Cash ain't the only consideration.
Look at it this way. Once you have:
parked at the big box
walked the 1/4 mile to the store
walked 1/4 mile to the aisle you need
made sure it was the right fitting (not in the wrong box)
made sure you got the one with the UPC on it
walked the 1/4 mile to the front
stood in line to use the %$*&! self-scan machines
waited for the cashier because you used a commercial card
walked 1/4 mile back to your truck
checked you truck to make sure it wasn't broken into
How much did that 39-cent fitting REALLY cost you?