But OP would be investing in more money by putting an ATM, I think that there's not a lot of people that would probably use their bitcoin to pay for such utilities, even with the discount, bitcoin is too precious to be used as a means of payment. The scenario that keeps playing in my head is what would happen to OP's gas station if the prices go down the next day and that next day is also the time for his restocking in supplies, how can he profit if not break even.
Many people use bitcoin as a means of payment, not everyone considers bitcoin as an investment. But accepting bitcoin for a gas station, I don't think it will work well. I mean, most of us want to fill up fast and leave immediately, waiting and jostling at gas stations is a regular occurrence, and it will take us longer to pay with bitcoin. If it were me, I would not use bitcoin in this case, and cash is still more convenient.
As for accepting bitcoin for business as just a way to attract more customers, I don't think anyone will hold bitcoins once paid, most will convert to fiat immediately.