It was between 5-6mm
Ouch, that is well outside the range of acceptable playability. The question of truss rod adjustment immediately comes into play - a quick tweak might solve the issue, depending on how the neck is now. Air transit can subject a parcel to temperatures of both extremes and anything can happen.
I don’t know if you’re an experienced player with all the knowledge under the sun or someone with less know-how than that. With that in mind, and assuming you were asking for suggestions about a course of action, I’d get the bass to a reputable luthier to check that the instrument is structurally sound and able to be adjusted to more comfortable playing parameters.


