Something like
these?
I've bought these from Uline, put the hard case in them, and I fill them as much packing material as I can. So far every buyer I've sold to has been happy with my packing jobs.
If anyone has used FedEx or UPS to pack their basses, I'd love to find out what that costs. I know it will be realtively expensive, but if the added cost is reasonable enough it might be good insurance in case of damage during shipping.