Septacycle--Wood blocks are one way to prevent damage in those areas, Plastic is another. 8 foot lengths of 1 inch white PVC tubing are another way to go if you ship a lot. Many times Home depot has small sawed off lengths laying around in the plumbing area and are free for the asking and they cut easy with a hack saw. Go to Home Depot and buy some 3/4" or 7/8" round Dowel Stock. Usually comes in 2 foot lengths for a couple $. Just saw off the correct length and attach that securely between the axle areas. You have to use something to prevent those areas from being compressed/distorted in case they lay heavier items over your box or as I wrote earlier, in case they drop your Box from a height of a second floor window. Or Ask a Bike Shop to sell you what they use, plastic devices to maintain the integrity, probably wouldn't charge you much. The round dowel stock idea from Home Depot is pretty cheap though and a 2 foot length would last for prepping a 1/2 dozen bicycles. BTW, in case of wanting to use extra packing material, I use styrofoam if its available and sometimes I use empty soda bottles of various sizes discarded in our recycling bins. These work just fine and are free and like styrafoam, they don't add much weight to the box thereby increasing shipping costs.