I'm sorry if this comes out as rude, but in Bitmixer's shoes, I'd do the same thing they did, and wouldn't feel guilty, or like I'm scamming you.
"Legally" (for whatever sense that word holds in the crypto-world) you donated 30btc to bitmixer.io. It was your own choice, and your personal mistake. God knows do I check my addresses thrice before sending, even after copy-pasting I can't help double-checking.

So in the end, if they don't refund you, you need to understand that YOU messed up, they're not liable for any of the damage you did to yourself.
Again, I'm sorry if this is rude compared to the other people sympathising, but this is what it is, you have to accept it and never make this mistake again.