I find an old package from amazon and stuff the cube in it with a few newspapers as cushion and maybe some bubble wrap, so it’s good to keep that stuff when you get it. I tape a piece of paper on it with their address. I then bike over to the post office and give it to them and ask to ship it for whatever option is the cheapest. Usually about 5-6 dollars US domestically.
Usually the buyer pays beforehand, and usually I have them pay for the price of the cube + $6 for shipping and I refund them if shipping is cheaper. If it’s a trade everyone pays for their own shipping.
Hope this helps, let me know if you have any specific questions.
Make sure you have plenty of bubble wrap or padding to protect the cube. Just wrapping it in some newspaper doesn't count. The suggestion about Amazon envelopes was a great one.