I think the commissions discussion might be off the track from the original question - the calculation of the returns.
Returns, loosely defined, is your percentage gained. You started with $1M, you ended with $8.4M, That's a gain of $7.4M. $7.4/$1=740%. In your calculation, you were counting your original $1M as a gain, which it's not.
Back on the commissions - we have a default commission in the model, so your backtest includes the $.03/share. You can read about the default and how to modify it in the docs.
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