I will be sure to read it thoroughly before ordering.
I agree that if you state in your TOS that prices are set in USD and refunds will be given based on that then you are being fair.
Think this sentence makes it clear?
If we have to refund, we will do so according to the value of the item in USD or EUR at the time.
Longer version:
In the case that we have to refund you, we will refund you in a currency of your choice (USD, EUR, BTC) according to the value of the item in USD or EUR at the time the order was placed.