From the info in this support email, I estimate that I will get hit with a 2.5% trading fee about 97% of the time when I deposit and trade in euros because the USD trade volume was 35x higher than the euro volume (last 24hr).
In practice the seller doesn't pay the additional 2.5% when a trade is carried out between different currencies. Instead, the sell order is posted into other currency markets at the original price +2.5%.
Basically, the buyer sees the sell order as +2.5% of the original sell order, but adjusted to their own currency.
Cheers