What is the best way to ship fenders from New Jersey to Kansas City? I have done Greyhound in the past (10 + years ago)
have heard not so good results as of late.

Use them all the time,no problems.

If you were to protect the sheetmetal (cardboard/wood), Fastenal may be another source? Not sure they will accept the bare fender due to the risk of damage or someone getting injured by the sharp metal.

Go to a body shop to get the cardboard. They just throw the cardboard away after getting replacement fenders, hood, doors etc.

Or a bike shop

i tried fastenal to ship a fiberglass hood from pa. to colorado. they won't do sheet metal because of damage risks. the lady [sam] i talked to was very nice, and although the shipping cost would have been low, she stated the damage risk is high due to the fact all their shipping is done loading by fork lift. engines, transmissions, rears, no problem as these are pretty rugged. i ended shipping the hood by bus. it was packed in an OEM late model hood box i got from a local body shop. i taped some foam strips to the hood and protected the edges with cardboard "angle iron" and pieces of industrial band saw blade protector. after packing in the box, i cut "hand holes" in several places to make it easy to pick up, then shrink wrapped the box. cost was $62 and change. the bus depot guy was surprised on the packaging. he said most just stick a tag on the part. got there in excellent shape.

I had a pair of 1965 dart front fenders to ship from central Oregon to northern Indiana, I got a empty windshield box form a local body shop and slid both fenders in it, separated them with other used cardboard and empty boxes and bubble wrap and foam from other packages
I ship them by Greyhound and they got their fine according to my buyer, that was around two years ago

X2 - find an auto glass distributor in your area. They used to use lightweight crates that were perfect size for shipping fenders in - they would throw them away. Not sure who you can ship with however anymore. Good luck.