That might be the problem, but it should show that amount from the beginning when you input the numbers. The way it works seems like a bait and switch, but I wasn't sure.
Yes, that's the problem. I just tried it to see how it works.
I tested it with exchanging 0.2 BTC and 0.002 BTC to USDT.
Currently, they give 3929 dollars for 0.2 BTC and 34.34 dollars for 0.002 BTC.
Given that the current price of bitcoin which is 20,040 dollars, the fee for trading 0.2 BTC is around 2% while it's around 14% for exchanging 0.002 BTC.