Worse still, if you increase your price, they would not come back, which make the first discount even less useful.
That's not a problem with spec work. That's a problem with the company. If they are happy for your work, and not trying to short change you anyway, then they'll not complain.
And when they recommend you to others, they're sure to mention that you're "dirt cheap." The fact that you do really solid work will be cited as an extra added bonus.