Obviously, most people would also panic if they transferred at least hundreds of dollars to a service, it hasn't gotten activated yet, and support did not respond in a timely manner. I think it's pretty normal, and all will be resolved when Mixero replies on this thread to the user.
When looking to transfer a large value of crypto assets, for example, thousands of $, investors should try transferring $ first to better understand how the service will operate. Similarly, users of mixers can also try mixing a small amount of BTC, for example, 0.002 BTC, to get a feel for it before mixing 250 BTC ^^
Mixero could also rely on this to offer a free mixing service to help users understand how Mixero works and the parameters of the mixing order, including fees and delay. Users would only be allowed to mix exactly 0.002 BTC and would only have to pay the transaction fee when sending BTC from their personal account to the designated address. After experiencing it and having enough information, they won't need to worry too much about Mixero service.