Flat fee

Flat fee is the simplest model. The customer pays one fixed amount every billing period. Quantity does not change the price.

Use it for plans like "Pro at $99 per month" where everyone on the plan pays the same.

Worked example

You sell a Pro plan at $99 per month.

What happens	Amount
Customer subscribes on May 1	$99.00 invoiced for May 1 to May 31
Renewal on June 1	$99.00 invoiced for June 1 to June 30

The quantity on the subscription item is always 1 for a flat fee price. Adding seats or usage does not change the charge. If you need the price to grow with a number, use per unit, tiered, volume, or stairstep pricing instead.

Settings

Setting	What it does
Amount	The fixed charge per period, in the price's currency.
Billing period	How often it charges: day, week, month, or year, times a count. "Every 3 months" is a quarterly flat fee.
Billing timing	In advance (default) charges at the start of the period. In arrears charges at the end.
Trial days	Free days before the first charge.

Edge cases

• Mid-period changes. If the customer switches to another price mid-period, proration rules decide the credit and the new charge. See the proration page.
• Minimums and maximums do not apply. A flat fee is already a fixed amount, so per-line guardrails have nothing to clamp.

In the API

Create a price with pricing_model: "flat" and the amount in minor units:

{
  "pricing_model": "flat",
  "pricing_config": { "unit_amount": 9900 },
  "currency": "USD",
  "billing_period_unit": "month",
  "billing_period_count": 1
}