I use u-haul "dish packs" 18x18x24 and cut the box down once I get both wheels inside. Well under USPS 108" rule, and protects the wheels beautifully.
stop at a tire shop. when i was at discount we would go through ten plus a day and would just throw them out every night
If you're shipping them with tires on I've always had good success in just taking two wheels and have them "face" each other separated by some thick cardboard. You can then strap/tape them together. 2x2 packs. Can't get the package any smaller! (that's what she said) I also have some wheel boxes but they are sized for 17x7.5-8" wheels only.
