For skis, use the free USPS Priority triangle boxes. Tape a few together. Strong, light, compact. I typically mail skis for about $16. Cheapest, best way I’ve ever heard. (Other ways easily cost $30-70.)
For bikes, use FedEx 3-Day Ground and use a free Trek roadbike box from a local LBS. But ya gotta pick the small ones. They’ll fit big bikes but there are a couple standard box sizes and this is the small one. It’s about 50 x 28 x 8. It costs about $45. (Other ways easily go up to $120.) You take off wheels, crank, seat/post, and loosen/rotate bars. Sometimes I undo my rear derailler, too. Tape pipefoam around frame tubes. Make sure you get the plastic hub-and-fork protectors along with the Trek box! Put extra parts — and tools — in cloth sacks and tape inside frame triangle. Tape chain into plastic baggy. Leave air in tires. I suppose there’s a size limit on the bike and maybe mtbikes are different, but there seems to be space left over so who knows.
