Vendor api payment conversion rate which exchange?

We are testing Turkish Lira - ETN payment via woocommerce. Seems to working smoothly, however i need to learn which exchange or the conversion point does the payment system api taken into consideration. I mean what if my customer calls me and says that “hey dude etn-try is x ETN right now why you are charging me x+1 etn?” whats the answer for this?

The only exchange with lira is sistemcoin so naturally that would be the exchange rate. Regarding the next answer it would be at the buy order at that immediate second for conversion (to guarantee fulfilment) and not the last buy/sell rate.