Charging a little at a time MINIMIZES the cost to the gas station. In actuality, the higher the price of gas, the less stations profit when people charge.
When someone charges, the station pays a transaction fee (small, like 30 cents) plus a percentage of the total, probably 2%. However, the markup on gas is typically in cents per gallon - often 10 cents.
So when gas is $3/gallon, a 15 gallon fillup for $45 costs the station 30 cents plus 2% of $45, for a total of $1.20. That works out to 8 cents per gallon.
When gas is $4/gallon, that 15 gallon fillup comes to $60. It costs the station 30 cents plus 2% of $60 for a total of $1.50. That comes to 10 cents per gallon in cost.
When gas is $5/gallon, the 15 gallon purchase costs $75. Cost to the station is 30 cents plus 2% of $57, a total of $1.80. That works out to 12 cents per gallon.
Remember the average markup is 10 cents per gallon. The bigger the gas purchase, the less profit the station makes - and can be a loss if their fees are higher than what I used as an example.
This is a major reason why gas prices shoot up quickly and drop slowly. The stations are trying to make up for the greater costs they paid when gas was higher.