Morality test, or a blessing (if you believe in that)?

Long story short: Went shopping today and was left with only $12 for gas, and my car was totally empty… I swiped my card at the pump, I was instructed by the screen on the pump to start pumping, so I proceeded to start pumping, looked away for a min. and when I looked back the pump said $16, I freaked out because I didn’t even have $4 to pay the difference… I then decided I would go inside and tell the cashier what happened, she looked it up on the comp and said it showed I didn’t pump any gas and I still had the $12 on my pre-paid card. I left the pump with $28 worth of gas for $12, but also a feeling that I should have said something more. Gas is so much right now, and my family is really struggling, The “free” gas couldn’t have come at a better time if you ask me, I feel blessed but also feel like I should have said something more…...