You could pre-screen for a certain grade or higher if you have more books to sub. Not sure what the minimum is now - seems like it was 50? If you don't have that many, possibly partner up or send them for a press to someone like @joeypost and he will combine them with others to meet the minimum I believe.
Just another option. But do the math, see what a 9.8, 9.6, 9.4, etc are selling for using GPA or just checking sites like EBay or Heritage through their free archives then calculate the cost of grading, shipping to and from to see where you may land.
Good luck.