Network has gone to shit a few days ago, you don't need to assume scam, there are problems all around because of congestion.
Contact support, maybe they will help.
i guess its common these days. nothing to worry then.
i still haven't get my few btcs from someone I've transact with its almost 4 hours now. i've trade with him 4 times already and this happen just today.
It's common if the sender (bitmixer in this case) didn't added enough fee to make the miner chose his transaction over other unconfirmed transactions. I'm pretty sure this is not a scam attempt, but an honest mistake by bitmixer.
I wouldn't say it's nothing to worry about tough, the receiver will not be able to securily spend the coins (create outputs with unconfirmed inputs is not a good thing to do) untill the transaction is confirmed, and if the transaction is eventually forgotten by the network (dropped by the nodes) the sender can (mistakingly) spend the inputs he used to create the forgotten transaction again, so the receiver will never receive his coins...
In the OP's case: it's very well possible bitmixer didn't add enough fees, so if the transaction gets dropped by everyone, bitmixer can use the inputs again, and the OP will be very unlucky. He can either rebroadcast the transaction untill it confirms, or he can contact bixmixer's staff, asking them to help him. Since he has his letter of guarantee, it'll be very easy to prove his point, and bitmixer can do a RBF or double spend the inputs, creating a new transaction with these inputs, but with a bigger fee...
