I'd be really disappointed, frustrated and angry. This was my bass, customised for me, not an off-the-shelf purchase that the retailer can send me a second one of. I'd be frustrated that it is the job of USPS to deliver stuff safely and they've lost it. Just lost it, with what seems like no apology or real effort or care for an item that is mine and that they have no idea how much value I place in the item.
I'd eventually get the money back and wonder if I wanted to go through the whole process again. Then the bass would turn up.