If you have more F.E. Perks than the price of fuel, any remaining Perks will carry over. You will only use enough to satisfy the cost per gallon. However, we are unable to do a $0 transaction at the pump, so the minimum charge would be 2 cents per gallon up to the maximum 20-gallon limit.
Here's a quick example. If you have $3 in FE Perks and fuel is $2.50/gallon, you would use $2.48 in Perks and only pay $0.02/gallon. You will then have $0.52 in Perks remaining after the transaction. The oldest (issued) reward would be redeemed first.
Keep in mind F.E. Perks are only good for 30 days from the date of issuance.