The simple thing to do is to message the freelancer and explain your situation:
Thanks for your work on the project. I was so appreciative of the way you helped us get this part done that I paid a $15 bonus. Unfortunately, I accidentally did that twice. Could you send me a refund for $15?"
99% of freelancers are going to be pleasantly surprised that you are paying a bonus, appreciative of this fact, and will quickly send you the necessary refund as soon as they are able to see that you did in fact make double bonus payments.
Alternatively, you could simply let this mistake go. Your freelancer will certainly not complain. He will simply think you are even more generous than he realized.
It appears that contacting Customer Support has indeed worked for you in this situation.
I'm just posting these notes to point out (for other clients reading this) that contacting Customer Support is not the only possible solution to a problem such as this.