I have a very old fashioned and economical answer that works well in Australia for my double bass. I send it via Greyhound bus, having wrapped it first in copious anounts of bubble wrap and bound with gaffer tape. I don't know whether this happens universally but I leave the 'parcel' at the bus depot, book it onto a particular service departure. It gets packed by hand and stays in the one cargo hold until its arrival. Bus companies here either charge by weight or volume. The volume charges are prohibitive, but the weight based carriage is cheap as chips. The interstate buses here average about 1,000 miles a day, so the most I've had to wait is 2 days. Regards, John