There is one JD part you should buy however, the full service manual.
Armed with this book, if you can work out which hand is left & which hand is right you will be able to dissasemble the mower right down to the last nut & bolt then reassemble iy with your eyes closed.
Seriously the JD service manuals ar worth the extra they charge for then because they are written for Joe Public, not greasy Joe the mechanic.
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If the $ 600 came from a JD dealer then send it into JD's head office with a "please explain".
While not as pedantic as small family companies, JD does take customer service & satisfaction quite seriously.
They stipulate the chargeable hours for each & every job and come down like the proverbial ton of bricks on dealers who profiteer.