A provider offered to do a test mix. That's good right?

Unfortunately not.

Providers that offer to do test jobs are not pros. Do not work with these providers.

You can always ask them for a 'post pay/when done' payment term which means they offer satisfaction guarantee.
You fund the job to SoundBetter and release the funds only if and when you are happy.
You can always contact us for a refund.
Good providers know this and insist on working this way.