Depends, if it’s prepay by credit card you only get billed for amount used regardless of how much you tell the cashier. If you prepay by cash you have to go get change.
Most pump transactions are just done by credit or debit card and you are billed for actual use.
To be slightly fair to the tech illiterate, there’s no sfp transceiver specified or shown in the picture… how the hell is someone supposed to plug that in?