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Well, this is a messy case. I once made my own mistake with bitmixer (for a much lower amount of BTC), it
WAS my own fault. It took awhile, and it also took some back & forth, but I did get my BTC back. They key thing (and this was well over a year ago), was to sign and verify that BTC address that I sent it from.
And THAT is one of the reasons why I signed on to their Signature Campaign. They resolved a problem, my error, for me in an acceptable way. YES, there was some back & forth.
But, keep in mind that they are likely the targets of scams every day...I have no idea if their policies regarding mistakes like yours (OP) or mine have changed.
30 BTC is a lot, I think that gentle persistence in asking for your
BTC is proper, and that if you follow their instructions completely, you will probably be made whole in the end. Keep working with (on) them, do what they say, and my guess is that you will get your BTC back.
* * *
That 30% offer you made to them seems EXTREMELY fair to them, IMO. There is almost surely some way that bitmixer could come up with the remaining 70%, there must me some controls that allow them access to THEIR own BTC. But, be persistent, polite and do whatever they tell you.
They have an apparently excellent business model. I doubt that they would want to ruin it with a bad reputation from an accident.
But, it really is important to carefully review transactions (which both OP and I did not properly do) and to follow instructions.EDIT:
One way to put yourself at less risk re accidents is send to send smaller amounts at a time, that of course means more times mixing, but that extra cost is near zero. And you get more familiar with the whole process.
Still, sloppiness can get one into trouble.
That is why I often lose in chess to players I normally should be able to beat... Good luck!