Every trade amount has to be a divisor of n. So you will never be able to trade the full amount you have. You will always leave some dust there forever.
The trading fees. 10% for trading BCH to BTC? That is just too much.
Hi there! We will be glad to explain what's going on.
Yes, there is some fixed minimum trading amount for every currency. It's called "Lot amount".
And yes, there can be some leftovers on your trading account, which do not exceed the lot amount.
But we believe it's better than having leftovers, stuck on orders forever (until you cancel them). That would be inevitable in case there were no lots. The total amount of supply just won't match with the total amount of demand precisely.
Speaking about trading fees - clearly this is some kind of mistake, we don't charge our users with such ridiculous commissions. Trading fee is 0.1%. Please try to refresh the page, this issue has to go away. Sorry about that inconvenience.