The consulting firm I used to work for used what was then called Rearden Best Travel. I believe it is now called Deem Rearden.

It worked fine it had an Expedia like portal that would should all options across airlines and marked what was in and out of policy.

For us overriding out of policy was fairly simple with a good reason (and it didn't require secondary approval) but I imagine it could've been made tighter.

No idea what fees were paid. They also had live TAs we could call with issues but we were charged a fee (that we could ultimately expense out but still it existed) if we booked through the live TA