First of all I reiterate its only some outlets and they seem to...

First of all I reiterate its only some outlets and they seem to be in USA .
I have never done it so I do not know but logically they would use the usd price per coin at the moment of transaction . If they are using both segwit and Lightning it would be either no fee or very little and instant . Bitcoin and litecoin for that matter using lightning are many times faster than mastercard or visa