Mixers have minimum amount of coins that they can mix. If you do not send up to that minimum amount for mixing, it would be considered as donation.
The minimum deposit can be 0.002 BTC.
Let us know the amount of the coin that you sent for mixing, and the name of the mixer.
Fair! But the question and point is...WHY WAS THIS NOT CLEARLY INSTATED BEFOREHAND?! it's not a given like beef is to burgers.
Have a look at my comment above to Cantsay. It was $7.