I very recently bought some things (presents for english family) on amazon uk with my french (euro) Mastercard. On cashout the option arose to pay in sterling with the Amazon Currency Converter. I did out of sheer convenience but it did occur to me that a rate of 1.13€ plus a commission of 2€ wasn't a particularly good deal when the real rate was about 1.10€.
I haven't explored whether it would be possible to pay in euros and let Mastercard do the conversion, nor what their rate would be.
Does anyone have any hard knowledge or practical experience of this situation? How do the sums work out? Is Amazon really cashing in on the convenience factor?