You are correct. Non-chain gas stations often make only as little as one or two cents per liter, and that's before you look at pump maintenance, inspections, periodical tank replacements/upgrades/liners and other costs.
Manned stations really need that shop otherwise they'd go bankrupt.
Chains make a bit more money but mostly because they can play longer games with stock and options on much larger volume buys.
Source: former gas station owner.
And yet I see in earnings that companies like BP and Shell make record profits over increased gas prices. How come that they do profit but the station holders not? Are shell/bp increasing the margin harder and eating the station’s lunch?