@occy is correct @Bushmanpat however there is one caveat, which is when you are using a broker with a tiered brokerage fee system such as Commsec you might be charged more. So if you're using Selfwealth or anyone else who charges you the same amount (a flat fee) for every trade regardless of the trade value then you won't be charged any further brokerage as long as your trade does not expire (remains valid in the system) exactly as @occy said, however if you're using Commsec or someone who charges a range of different brokerage fees depending on the value of each trade, then you could be charged more if the total of your order exceeds the range for which they charge $20 (i.e. what they've already charged you).
I use both Commsec and Selfwealth, so I'll give you a Commsec example. If I placed a trade to buy 1,000 FMG @ $20.45 each, and got just one share today, they would charge me $10 brokerage (their fee for trades up to $1,000 for customers who use a CDIA account for settlement - a Commonwealth Direct Investment Account - so their preferred customer brokerage fee). If another 47 of those shares got bought tomorrow on the same order, which is still open and in the market, I wouldn't get charged any more at that point because the total of my order (48 x $20.45) has not exceeded that $1,000 limit that qualifies me for that low $10 brokerage fee. However if instead 50 shares went through tomorrow (instead of 47), they would charge me another $9.95, because their brokerage fee for me for trades between $1,000 and $10,000 is $19.95, and I've already paid the $10, so I'd get charged the difference ($19.95 - $10 = $9.95). If the entire order got filled tomorrow, that would come to $20,450 (1,000 x $20.45) and their charge for that would be $29.95, less the $10 I've already paid today, so I'd be charged another $19.95 to make the fee up to the $29.95 brokerage fee that they charge for orders over $10K and up to $25K - see here: https://www.commsec.com.au/support/rates-and-fees.html
So, you'll always get credit for any fees you've already paid, but you may have to pay more if the total of the trade moves it up into a higher fee category.
Just remember to ensure that the order doesn't expire - make sure the expiry date is extended preferably for a month, and you might have to extend it again if the order hasn't been filled within the first 3 weeks. You can edit the price and the order expiry date at no cost. The only time you are charged is when actual trading occurs, i.e. when actual shares are traded, not when you place an order or modify an order.
Short answer is that if you use a flat fee broker, you won't have to pay any more brokerage, but if you are using a tiered brokerage fee broker (whose brokerage fees depend on the amount of the trade in dollars), then you may have to pay more, as I've explained. Hope that helps.