I had purchased an AmpereTime battery that was bought in the US and shipped to me by UPS. It's a *sealed* battery (no visible seams), and had a bad BMS; AT gen'd the shipping label for me, and UPS picked it up and shipped it back to the vendor in the US ... nobody had any concerns.
This is not your scenario at all, except to both point out one method by which LFP batteries are getting shipped (sealed battery case, and vendor gens both shipping and return labels, all in the US), and to ... possibly ... suggest that you "seal" these up in a plastic abs container of some kind (a shipping container, possibly), and then try shipping them. It might cost an abs case that you weld shut, but perhaps this is less than being out the cost of the batteries.
Return shipping costs, and who pays, are a different can of worms ...
Shipping overseas ... another can of worms ...
It looks like, for DIY battery builds, we're going to have to find vendors who ship us X amount of pieces, and then if a bad battery, they just ship a replacement, and don't expect the return of the bad item (which gets recycled) ... this happens with some kinds of Amazon purchases; you can also try arguing this with your battery vendor.
For the rest of us, *specify* this, in writing, before battery purchase (if possible)?