Steps to reproduce
Enable Price Extras and add an extra price to a listing (e.g. 1000 usd for renting a car + 200 usd as an extra for insurance)
Enable Woocommerce Payments
In Woocommerce Payments enable Currency Conversion and add other currencies (in this case we are going to add YEN and USD).
Actual result
When a customer does the checkout and pays, he can choose the preferred currency. If I choose as a customer, USD, I need to pay 1000 USD + 200 USD as an extra = 1200 USD
If I decide to pay in YEN, I get this: 141.000 YEN + 200 YEN as an extra = 141.200 YEN. If we convert those YEN back to USD, you can see I am actually paying 1001, 53 USD instead of the 1200 USD showed above.
Basically the normal price gets converted but the extra stays at the same value (200 stays 200 whether it be EUR, USD, YEN or anything else!).
Scammers could use this for paying in a different currency and pay way less than the due amount. The booking anyway, would result completed, while the missing difference should be paid by someone else (not the scammer!).
Expected result
BOTH price and Extras get converted correctly so, the expected result in the example above should be: 141000 YEN + 28000 YEN = 169.000 YEN which is 1200 USD (CORRECT!)
