No in these days of computers there is no extra charge for a limit order. But just remember that a stock can trade right THROUGH your limit if it gaps to the upside or downside! You might want to place a stop loss order — which becomes triggered as a market order if that price is reached. You may wind up selling at a much lower price than your stop loss, if the market is moving quickly to the downside.
As for the brokerage firms making money, payment for order flow is just a very small fee that the large trading firms pay retail brokers to direct orders to their “book” to execute, so they can make the tiny “spread” between bid and ask. You’re always guaranteed an execution at the best price available on the national market system. This is just a sort of hidden profit center for retail brokerage firms.