Bitcoin MIxer (Tumbler) (mixertumbler.org) are scammers - they happily took my bitcoin, and I got bugger all back! Avoid them like the plague!
I think there may have been a previous domain name (mixertumbler.com), and I have no idea as to whether or not they were legit when using that domain.
I had never heard of mixertumbler(.)org nor seen them advertised here. How did you find out about them? There are many mixer services with a long history and reputation, so I see no reason for you to risk your money with some unknown service.
Mixertumbler(.)com had an
ANN here, but the site, as far as I can see, has been shut down for a long time. These two domains are not necessarily related despite their similar names.