Parcel with SOD, presumably.

Or a very thin box. Rather than speculating I would prefer to wait for the OP to enlighten us, if they so choose.

I suspect they have gleaned the information they were after and haven't bothered to read the last 15 posts.

Not massively important, but I'm going to go ahead and presume registered letter - if you add SOD to a parcel, extra cover still costs more per $100 (including the first),  so maximum compensation would still be $50, if you add registered to a letter, it includes $100 extra cover, which is what OP received.