"There may be a misconception when you hand me a $100 bill, most of it to pay for a tank of gas," Tempero said. "Out of that same $100, I might get 50 cents of it."

Percentages determine profit on other retail items, Tempero explained, but "with gas it's so many cents a gallon. When the price approaches $4 a gallon and you're making a 3-cent margin ... "

Well, that's when credit card purchases come into play.

For one thing, Tempero said point-of-service fees charged by credit card companies can cost a retailer as much as 9 cents per gallon.

"You can go backwards 5 or 6 cents on that gallon," he said.

It can also take time for that credit to cycle back into the business's account - and if the credit's not yet there, then you need the cash to order more fuel.

"It can make a $20,000 to $30,000 difference on your invoice, just because of the credit cards," Tempero said.

And that's what happened last week. When the Y Quik Stop ordered more fuel a couple of days too early, Tempero said, there was an electronic funds transfer due of $23,000.

"We were not able to pay that," he said.