Dollar Dbl

If the tax is 7.5%, what is the total cost of a dbl whopper with cheese that cost 5.70.?
I seldom if ever look at the receipt taped to the bag from BK but today as I sat in traffic, I was astounded that they charged .40 for the cheese. I also noticed $7 tendered after I had given the cashier a $10. I called the manager and he said there’s a 7.5% tax charged for fast food. Now, after he did his math he said I had actually been undercharged; he came up with 6.26. I paid 6.16 total. I did some calculations and came to the conclusion the tax(.46) is closer to 8%. The manager also said the register computer automatically rounds up/down the cash tendered to the next full dollar amount. I live in NC and the tax on food from the grocery is 7%. Guys look, I know its only a few pennies, but I hate the idea of being ripped off even for that amount so humor me if I’m wrong, okay.
$5.70 x 1.075 is $6.13 to the nearest penny. Simple math. What’s their problem?
DA BRONX RUFF RYDERS RIP MONEY MIKE “DOLLAR” 2010