Don't think this is an issue btwn Fedex and UPS. Both screw up from time to time, and a lot depends on who just happens to be handling the packages. For years I hated Fedex and loved UPS. Then they both changed delivery guys and guess what. I'm sure that happens all the way down the line from the person 1st taking the package, to the sorters, packers, drivers, etc. I think it's kinda silly debating which company is better.
As for the bass itself, my red flag popped up as soon as you said they told you "of course" they detuned the strings. I've had countless basses shipped to me, and have shipped many basses myself. I never had a single one of them detuned, never expected them to be detuned, never thought about detuning them, and actually would think that would be more harmful than helpful in the shipping process. Their response of "of course we did" seems like them wanting to cover their butts bigtime.
I've only had one bass delivered that was damaged and the company sending it took care of it, no problem. A piece of the body looked like it was hacked away. I believe the vibrations from the truckride alone could do all kinds of things if theres a slight flaw in the instrument. I don't entirely trust that the person selling it to you didn't pack it as such. It very well could have happened some time btwn picture time and delivery and they just said screw it, we'll send it blame delivery. But it doesn't even really matter. What matters is that you get your money back, and there is NO FREAKING WAY I'd accept their store credit BS. You wanted THAT BASS which came to you busted through no fault of your own. I say they either replace the bass or give you your money. Store credit is BS.
Didn't realize how passionate I was about this whole thing till I started typing.

Waiting for the outcome. I've a feeling it will be fine.