Edit: $3 for delivery generally and they're using F250 trucks. Call it 1.5t in the bed or about 400 gallons plus tank. They probably have a contract with the local bulk terminal so if you're paying 2.55, they might be paying 30% (no idea what this margin is) less or so. @400 us-gal if they make 30% on the gas plus $3 per 15gal tank, revenue would be in the $380/load range for an average of 26 tanks of gas per load.
Edit2: regarding the loss leader strategy, they still have to truck the gas to the station which adds overhead. By going direct to customer (again, assuming they're loading at the bulk terminal) they remove a couple middlemen who's position they then take.