The usps number is a delivery confirmation number. It is not intended as tracking. It is intended as proof the item was delivered.
If you call the usps, especially your local branch if you know them, they can take that number and see if it was addressed correctly. Their software is better than what is on the web. Parcel post travels from station to station only when there is room. It moves in short chunks. And basses are big and frequently don't make the cut.
But, the good news is that it will probably turn up. I had someone send me a bass from washington state to tennessee and they mis-addressed it to somewhere in NC. It took me over 3 weeks to get it, but it was eventually intercepted and sent my way.
If you have it insured, it should be ok. Of course, there are horror stories about all shipping companies, but if you packed it well, it should be fine.