Then the motor6ds must be okay, as @yO_eyDev said, make sure you are using a Motor6D to attach the box to the NPC. Motor6D basically makes it animatable, let me know if you need more help.
No, you don’t get rid of the animation, Motor6Ds are the ones that make the animations work. Basically, all you got to do is, delete the weldconstraint and replace it with a Motor6D. A motor6d has pretty much the same properties as a weld, it just makes instances animatable. For example, your character is rigged by motor6ds and thats how all the animations work in it.
You can insert a motor6d from explorer and define its part0 and part1, or you can do it from a script, works both ways.
I see, did you make sure you got rid of the weld constraint? If yes, please double check the animation, also set cancollide to off in the properties tab of the box instance.