Do you say that you offer a set number of takes in your gig? If not, I’d simply let the buyer know that typically you only include one take per order, but this time included a second one as a courtesy/free extra. Then let them know that you will have to charge for more.
If you do offer a set number of takes, say something similar but also refer back to your gig description.
Also, if you do only offer one take but usually send more, you may want to reconsider this. I sometimes include freebies with orders. These can include everything from extra word counts to fee images and video shorts.
Sadly, this often results in buyers asking for more free stuff. i.e. I include a free animated gif for use as an ad on Facebook, and a buyer asks me to turn this into a 30-second explainer video. These situations are also very difficult to navigate, as most buyers don’t really understand (or want to understand) why they should pay more.