Full Mastering is for mixes that didn't come from us — your own work, another engineer's, or a session from years back you're finally ready to release. The extra cost over Light Mastering is the evaluation: listening to a mix cold, figuring out what it needs, and deciding how to finish it.
One thing we'll always be straight about — mastering finishes a good mix, it doesn't repair a broken one. If your mix has problems that mastering can't solve, we'll tell you before we charge you, and you can either revise and resend it or have us fix it at normal mixing rates. Nobody wins when a master gets pushed onto a mix that wasn't ready.