What is the best way to ship a Bundt cake in the mail?

Answer 1

The Box
Choose a box with enough room for cushioning material around the contents. Sturdy paperboard or corrugated fiberboard boxes are best for weights up to 10 pounds. If you are reusing a box, cover all previous labels and markings with heavy black marker or adhesive labels.

Where to Find Boxes
You can purchase boxes and tubes of various sizes at most Post Offices. Express Mail and Priority Mail boxes are available at the Post Office for items sent using either of these services. While you are not required to use the free packaging for these services, you must use the address label provided by the Post Office for Express Mail..

Cushioning
Place the cushioning all around your item or items. You can use newspaper, styrofoam "peanuts," bubble wrap, or shredded paper. Close and shake the box to see if you have enough cushioning. If you hear items shifting, add more cushioning.

Placing an extra address label with the delivery and return addresses inside the package will ensure that the item can be delivered in case the outside label becomes damaged or falls off.

Mailing Fragile Items
Use foamed plastic or padding to protect your items, placing the cushioning inside hollow items as well. Mark the package "Fragile" or mark "Perishable" on packages that contain food or other items that can spoil. Careful packaging is the best way to safeguard your valuable items against damage.
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Answer 2

You don't say how far you're shipping it - if possible, I'd say ship it in the pan you baked it in - that way it won't get all broken up. Or maybe wrap it really good in Saran Wrap, then pack it in air-popped popcorn (no butter or oil), that's a good "shock absorber".