Is it possible to pull your trailer up to where the spa goes? That's what my dealer did- but their trailer converted to a spa dolly.
That way you can minimize the amout of distance you have to carry the spa off the trailer.
I've heard some people use several PVC pipes of maybe 4" diameter, and just roll the spa on those pipes, taking the back roller and moving it up to the front as the spa moves forward.
One thing to consider- if the movers drop or otherwise damage the tub they are responsible for any repairs.
If you drop or damage, you pay, and it might wipe out any savings.