Wheel shipping and boxes, a better way.
Tire and wheel stores give away wheel shipping boxes for free.
They're glad to have someone take them off their hands instead of needing to crush and bundle and recycle them. These usually have all the cardboard inserts and separators in them, so no extra padding or newspaper needed.
Ship from a local UPS or FedEx depot, or the USPS. Don't underestimate the USPS, you can send a 25# 18x18x10 box for $23 - $28 depending on location. All local UPS stores add a markup. UPS depots don't. FedEx is pot luck, generally cheaper than UPS.
Two boxes taped together shipped as one package ships for less then separate packages.
If you bought new wheels, reuse those boxes to ship your old wheels back out.
There is also a mid-sized U-Haul type moving box that takes a 17 or 18" nicely. Put two wheels inside two of these, one on the bottom and the other on top as a cover.
Forget about buying used tires unless they're made of gold. Shipping with the tire on doubles the weight, and makes shipping cost impractical compared to what you can buy new tires for locally, whether new or used.
Also, get tread depths or tread remaining measures _Above the wear bars. Otherwise, if the seller says half tread, that means only 2/32 or less above the wear bars. To most of us, that tire is nearly worn out already.