As jj94tt suggests, the amount you were charged for parts would appear to include the cost of new cups. The three parts in question add up to $677 based on current prices from Jacks Small Engines.
Having replaced a PTO clutch in my own G series tractor, I cannot imagine not replacing all three parts. While I had mine torn down, I did every seal, and the clutch discs as well.
You mentioned earlier about belt problems with a 50" deck. Please tell me more? I have 1995 production G tractor with a 50" deck, purchased new in the spring of 1996, and have never had any unusual belt wear or failures - except the one time I tried to use a non GRAVELY belt.
My deck has had one complete going over to rebuild spindles, replace idlers, etc., but in 23 years and about 800 hours it is only on its 4th belt, and one of those was my fault.
My deck has also been upgraded with dual blades, hybrid spindles, and ball bearing anti scalp rollers.
The 50" deck can be "picky" about belt length. And, I have seen newer decks with the tension idler installed wrong. The newer version uses a fabricated tension arm, and has two different mounting locations for the idler, on for the 40" deck, one for the 50" deck.
Incorrect belt length, or incorrect idler location, will cause it to eat belts.
If you have never taken a look, you can read about the long list of improvements I made to my G series tractor here:
I added steering brakes, an aux pedal to kick the tractor into neutral, down pressure assistance to the snow dozer, brighter headlights, lubricated tie rod ends, and much more.