Your agreement with Enterprise apparently stated that you needed to return the car with just over a half tank of gas. You said you returned the car with just over a half tank of gas. You met the terms of the agreement you had with Enterprise, this even if you used a little bit of gas to drive 30 miles.
Enterprise was not concerned enough to charge you for the gas so you should not be concerned about it.
That said, if this is one of those worries that is going to overwhelm you and you think you will not be able to function unless you pay for what you used, then you can mail Enterprise a check for an amount covering the 2 gallons of gas you might have used (What is gas out your way? Three bucks or so a gallon?) and you can enclose a short note saying that you drove 30 miles and failed to replace the gas you used. I am sure Enterprise will be surprised and puzzled but, if it lets you sleep at night, just do it.
Good luck to you, AdviceFreeUser.