Wiktionary
flat-rate
a. (UK) Relating to a flat rate; with the same amount of money required of everyone
Usage examples of "flat-rate".
This last could be deduced from the datum that they had compounded to pay a flat-rate tax per year instead of income tax, though the sum appeared absurdly high considering there was no industry bar handicrafts (not available to wholesalers).
The Constitution specifically required that any income tax be flat-rated and limited to a maximum of eight percent of gross income except in time of emergency.