.. WTF? "we want all but the dangerous bit back"
Makes total sense to me that posting or couriering faulty Li-* batteries isn't something a responsible company would encourage.
UPS lost a 747 over Dubai due to a Li battery fire, the delivery industry got a lotttt more careful about Li batteries after that.
OTOH they almost certainly don't want a bunch of defective batteries coming in the mail when the ultimate resolution they'd perform is the same as you'd perform locally.
Why don't you just dispose of the battery yourself, and snap send you a new battery? And you keep the drone?