I remember on other platforms there is a notification that if you send below the minimum amount it will be considered a donation.
It's exactly the same with Sinbad, however, that information can be found in the FAQ section, not the main page. And we all know how much people like to read about a service before using it... they don't.
The main page has information about the minimum mixing amount being 0.001 BTC. Perhaps the same warning could also be added there. Any deposits less than this amount will be considered donations to Sinbad that can't be used to initiate a mixing round.