Courier-proof packaging without a hard case is possible, I've done it many times. It takes time, thought and care, three things that some people are unwilling or unable to devote to the task. The basic premise is to put the bass in a gig bag and then wrap it heavily/pack it tightly in as much bubble wrap, foam, scrunched up newspaper, polystyrene bits as fills the outer box to make the whole package tight without risking bursting the outer box. The trick is to make the whole thing one monolithic object - absolutely nothing loose or moving inside. Once that state is achieved I am confident of taking on all challenges when it comes to accidental damage.
This is just my opinion and although it means little statistically given the small sample set (circa 10 parcels) I have 100% success rate in getting basses to people in the condition I sent them in so as far as I'm concerned I seem to be doing something right.
Most recently I have been using Interparcel/UPS but have in the past used Parcelforce (until I found out about the insurance limit in the teeny tiny print for musical instruments, hard case or otherwise).