The 0.0003 additional services fee, is it different added to the main service fee?
Yes. The fee is 0.7% (slow) to 4.7% (fast) which you can adjust using the fee slider + 0.0003 BTC as the service yes.
And the both wit deducted from the service fee at once?
Yes.
And why the additional 0.0003 mode service fee is added to the main service fee.
That is how some mixers are. You pay fee for the company to make profit from the good service they are rendering.
And I really like to know the function of 0.0003 to the service fee or are they the same? And the service fee is it deducting from the 0.002 mixing amount or I will add extra amount for the service fee? Because as you said, if the amount is less than 0.002 BTC then it becomes a charitable donation. Buy if the person send 0.002 BTC and from the amount you removed your service fee amount and the left over will not be up to 0.002. will the remaining amount be send to the destination address?
The fee would be deducted from 0.002 BTC, but make sure you do not send any amount less than 0.002 BTC because it would only be seen as a donation.