I let National refill the tank for a rental in Las Vegas two weeks ago. It was 1.69/gal plus state taxes. Pump prices were 1.82 at the stations near the airport. The tax situation (at least in NV) is funny, since the usual pump price includes all taxes, of course, while the price on the receipt has a separate line for the state taxes and an asterisk indicating the line items being taxed. Thus the 1.69/gal excludes state taxes while the pump prices at 1.82 include them. The effective National price/gal worked out to about 1.83/gal, so it was competitive with the prevailing prices.
The National agent who uses the scanner/computer/receipt printer estimates the amount of gas needed to fill the tank (in gals) and you get charged for this amount. I had used about 1/4 of a tank and was charged for 4 gals (Toyota Camry). This seemed reasonable to me.
The convenience was great. I would easily pay an extra 0.10/gal for the service. I would never pay the 3.50/gal price in the past and so always had to allow time to find a station close enough to the airport for the gas gauge to still read full when the car was dropped off. I will continue to let National refill the tank until I feel I'm being ill-used with this process.
Ralph