I remember we discussed this before. What exactly is the fee that you charge? Can you link to a few examples of Mercury withdrawal transactions so we can see what exactly they look like?
The withdrawal fee is currently 0.5% of the coin value - this is the only fee - unlimited (within the timelock constraint) swaps are free.
We are working on a new feature where the fee can be paid upfront before deposit, either via a LN payment or separate bitcoin tx, enabling users to have no on-chain link to mercury.