Just to be clear, they're not talking about filling up his vehicle, they're talking about topping off their <1L camp stove, which they may have only used 20% of but want it to be full for the upcoming nights of camping. It's feasible they both only needed $0.25 of fuel to top it off, and also that the extra $0.25 of fuel could make the difference over a 4-6 night stay.
As for sliding a quarter, often gas stations have a barrier between the customer and the proprietor, necessitating one party or the other to do some sliding of coins at some point during the transaction. GP could have placed the quarter near the slot without sliding, but the clerk would still have to reach past the slot and slide it towards them.
20% of a liter is $0.25 -- There are cases when you're traveling/camping that you may have used 20% of your 1L can but need to refill it for the next leg of camping.
If I was buying $0.25 of gasoline I'd be slightly weird about it too, at least to acknowledge / show awareness that the situation is indeed strange. Doing it in the same non-plussed manner as I buy $20 of gas would seem more awkward to me than adding in a touch of bashfulness. Adding additional social cues that something is unique about the transaction can also head off clerical errors like processing my request as "oh he clearly meant $25, not 25 cents".
Just to be clear, they're not talking about filling up his vehicle, they're talking about topping off their <1L camp stove, which they may have only used 20% of but want it to be full for the upcoming nights of camping. It's feasible they both only needed $0.25 of fuel to top it off, and also that the extra $0.25 of fuel could make the difference over a 4-6 night stay.
As for sliding a quarter, often gas stations have a barrier between the customer and the proprietor, necessitating one party or the other to do some sliding of coins at some point during the transaction. GP could have placed the quarter near the slot without sliding, but the clerk would still have to reach past the slot and slide it towards them.