it is hard to say where the problem is without knowing what you are really sending them.
i don't see any coin named "ratex" on their exchange to give a proper example but lets say there is and its price is 0.0001
BTC you have to buy at least 1 to be above the total amount. 1 * 0.0001 = 0.0001
BTC > 0.0001
BTCif that didn't work then add the amount of fee to it.
0.0001*0.2% = 0.00000020
BTC and make the request again. it would look like this:
pair=ratex_btc&type=buy&rate=0.0001&amount=1&method=Trade&nonce=YourNonceHere
with your second numbers the request should be this (make sure you are not mixing the amount and rate):
pair=ratex_btc&type=buy&rate=0.0009&amount=2&method=Trade&nonce=YourNonceHere
if that didn't work then contact their support!