I can test your service.
You are charging much less than other comparable shapeshift-like services. The two taker fees you would need to pay on an exchange would eat up the majority of your profit.

I might suggest using USDT, or another stablecoin for USD based checks -- a person can fund a USD checks only with the stablecoin, and they can only be redeemed for said stablecoin. Similarly, BTC and ETH checks could only be funded and redeemed with their respective currencies. This would remove any exchange rate risk from your business, and you would avoid inevitably having to charge higher fees.