Stamp Duty An ad-valorem or flat rate charged upon certain documents.

对一些文件按其价值收取的收费。

youdao

Interest is charged at a flat rate of 11%.

利息按标准比率11%收取。

《牛津词典》

Fees are charged at a flat rate, rather than on a percentage basis.

费用按固定的费率收取，不是按百分比。

《柯林斯英汉双解大词典》

When electricity first came here, dos Santos says, the utility charged a flat rate to entice people to go on the meter.

FORBES: Life In the Favela: Gatos Vs. The Working Class

That seems to exempt long-distance service that is charged at a flat rate per minute, as most is these days.

FORBES: Phony Tax

To process the payments, businesses must rely on third-party merchant service providers (MSPs), which offer a dizzying array of rate structures that range from a flat rate to a percentage of the amount charged to the card.

FORBES: Written by Robert Ziegler