Gas Scam ?

So, I went to one of the cheap gas stations I like to patronize. "Jacks Gas"- It's a small station, I think it's still owned by the same guy that owned it when I was in college. Maybe he's got two more stations. He's just not part of a chain. I try to "Shop Local" whenever I can.
However, I'm not a real big spender . With the price of gas these days, I'm pretty careful with the trips I take. I can usually get 3/4 of a tank for $20.00. SO, I usually fill it with $20.00. I haven't said "Fill it !" To a gas station attendent since I had the van, and I was heading to New York.
So , tonight I went to get $20.00 of gas at Jack's . I said "$20.00 of regular and I'm paying cash". He unlocked my gas cap and put the nozzle in. I think I was checking my voice messages or something and when I looked back to see what the total was , it was $27 ! I started yelling "Hey !Hey!" "Buddy! I said $20.00 ! Hey !" SO the guy says "You said 'Fill it up '!" I said "No ! I said $20 . 00 bucks and I'm paying cash!" He said "No! You said fill it up ! " I'm sorry the little prick doesn't have the right job skills to work indoors, but I didn't "say fill it up" . It ended up being $28.00. I hadn't budgeted for that AND he 's got an attitude !
Though I did get the amount that my $28.00 paid for, which I didn't need right now, I'm wondering if this is some sort of scam to get the evenings totals up. He's not skimming profits . He's just putting in more gas than you ordered and then getting belligerent about getting paid for it. At $2.99 a gallon, I'm guessing everybody is filling up more judiciously. But that left a bad taste in my mouth. I really wanted to cry. I work a shitty job, too. Why did he have to be such a prick ? I don't need that kind of treatment .
Prick.
I'm not spending any more of my money there. I work too hard for it.