Kind of like retailing gasoline, there is no profit in it, so they sell candy, cakes, coffee for their profit margin. Speedway is the master at making their money on accessory add on foods...……………….
When I had my service station it was right at the infancy of convenience stores. I had a Frito Lay display rack from one of the Frito Lay drivers whose trucks I serviced. I made more money on a couple bags of single serving chips and two cans of pop from my own pop machine, than I made on a fill up of a car………...
Oil company was always on me to drop the price of gas, told them if they wanted to sell it to me at a reduced price, I'd be happy to do that. That didn't sit well with them however...………...LMAO. They were incessant that I'd make it up on the volume I'd be selling...….

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The garage was where the money was made, but in the late 70s early 80s the oil companies were just beginning the phasing out the full serve gas stations.