I got a pair of wheels delivered recently. They just came in a cardboad box, the size of a wheel (but square obvs) with some cardboard packing pieces. Skewers removed. It's a sturdy box (has two layers of corrugations not just one). So sturdy I've not got around to attacking it and squashing it into the recycle bin yet.
I also once received a wheel by post, again in a box, but the sender had attached square packing pieces either side of the wheel with string so it effectively became a square wheel that fitted neatly into the box.
So if posting a wheel I'd try to obtain an appropriate sized box (LBS may help, also next to the dumpster at the back of
Halfords - I got a great cardboard bike box there once) or failing that improvise something out of other cardboard boxes and duck tape.