What is platform fee rate, and how do you measure it?

WITH monthly AS (
  SELECT
    platform,
    date_trunc('month', ordered_at) AS month,
    SUM(gross_amount) AS gross_sales,
    SUM(refund_amount) AS refunds,
    SUM(commission_fee) AS commission,
    SUM(processing_fee) AS processing,
    SUM(payout_fee + fx_fee) AS payout_and_fx,
    -- Small orders where a fixed per-transaction fee dominates are
    -- where the effective rate diverges most from the rate card.
    COUNT(*) FILTER (WHERE gross_amount < 10) AS small_orders
  FROM creator_orders
  WHERE status IN ('paid', 'partially_refunded', 'refunded')
  GROUP BY 1, 2
), rates AS (
  SELECT
    platform,
    month,
    gross_sales,
    refunds,
    small_orders,
    commission + processing + payout_and_fx AS total_fees,
    100.0 * (commission + processing + payout_and_fx)
      / NULLIF(gross_sales, 0) AS effective_rate_pct,
    100.0 * commission / NULLIF(gross_sales, 0) AS commission_pct,
    100.0 * processing / NULLIF(gross_sales, 0) AS processing_pct
  FROM monthly
)
SELECT
  month,
  platform,
  ROUND(gross_sales, 2) AS gross_sales,
  ROUND(total_fees, 2) AS total_fees,
  ROUND(effective_rate_pct, 2) AS effective_fee_rate_pct,
  ROUND(commission_pct, 2) AS commission_pct,
  ROUND(processing_pct, 2) AS processing_pct,
  -- Rate drift in percentage points, not fee growth: this is what
  -- catches a 30% -> 15% tier flip or a lapsed discount program.
  ROUND(effective_rate_pct - LAG(effective_rate_pct) OVER w, 2)
    AS rate_change_pp,
  ROUND(gross_sales - refunds - total_fees, 2) AS net_to_seller,
  small_orders
FROM rates
WINDOW w AS (PARTITION BY platform ORDER BY month)
ORDER BY platform, month;