noun
flat-rate system; fixed-rate billing
Refers to a billing or pricing model where a fixed amount is charged regardless of usage, common for subscriptions, utilities, or services.
このサービスは定額制で、毎月同じ料金です。
This service is a flat-rate system, so the fee is the same every month.
定額制のプランに切り替えると、
If you switch to a flat-rate plan, you can use it as much as you want.
Compound of 定額 (fixed amount) and 制 (system).