I can fully understand your concern!
I also purchased my Raptor Flip Up through Lowe's and they delivered as well. But unlike your situation, everything was fine. I've had it since 2016.
Now what you are referring to is the mowers "Hydraulic Expansion Reserve Tank". This small tanks fluid level should be at the "Full Cold" line only and definitely not above that line. It should never be above that line. The FULL COLD LINE is located at the bottom of the tank.
The best time to check that should only be when the mowers engine is cold (or in other words the mower has not been on or running at all).
Now the only other time that this specific tank will show a fluid level above that line is when the mower is running or right after it had been shut down, of course the engine being hot, etc. But after a total cool down of the mower, that level should drop back down, either below or right at the Full Cold line.
With all that being stated. Double check your owners/user manual and read up about that. It may give you something a bit different but I doubt it.
It appears that you just might be between a rock and a hard place on this. First of all, I don't think Hustler would ship an mower with that tank full of fluid? But again I could be wrong about that.
Second, someone over at the Lowe's store that you purchased it from may unknowingly added fluid to that tank? But again, it's hard for someone to fess up about that. "IF" they did, lets hope that they used the correct fluid called for in the owners/user manual (which on mine calls to use SAE 20W50 service motor oil) and not Hydraulic or Automatic Transmission fluid. Because either of those fluids should never be used!
So if you just want to leave it and not deal with it your options are to either contact Lowe's or Hustler directly and explain the situation to them and see what progress'es out of that mess. Both are more than likely to lay fault at each other to get out of it.
But not all hope is lost!
What you can do is remove some of the fluid from the tank until it reaches right below the FULL COLD LINE.
To do this simply get one of those syringes that is used for basting turkeys, remove the tanks black top, insert the syringe and start removing the fluid, again until you reach the FULL COLD LINE.
Once done, then you can try out your new mower and give it a go.
The choice is yours on what you want to do, this is just my $0.02 on it all.
Oh and go back and read all about your mower before you actually start to use. It's helpful, also there should be a 1-800-# to call if you have any particular questions.
Good luck and I hope everything turns out well for you.
Keep us posted on the overall outcome on what happens.