Robinhood raked in a record amount of revenue from customer trades in the first quarter of 2021, as the retail trading juggernaut nears its public debut.

The millennial-favored stock trading app collected $331 million in payment for order flow – the money brokerage firms receive for directing clients’ trades to market makers – in the first quarter of 2021, according to a recent Securities and Exchange Commission regulatory filing.

The pioneer of “Free trading,” Robinhood’s business model hinges on the back-end payments, in the absence of commissions.

Market makers, such as Citadel Securities or Virtu, pay e-brokers like Robinhood for the right to execute customer trades.

Robinhood – which is expected to go public on the Nasdaq in the first half of 2021 – made $133 million in payment for order flow from equity trades, while $198 million came from options trading.

The boom in order flow coincided with record retail trading activity and new customer accounts across the industry.

Main Street argued that it gives Robinhood reasons to incentivize more trading.