If I was silly enough to offer a buyer a choice of not registering a $70 item and I didn't pay for registration myself and the parcel went AWOL, then I would wear the loss as a cost of being silly.

Surely if it's not an auction you would build the registration cost into the item or postage price, thereby not giving the buyer a choice. If it's an auction and it went for more than you anticipated then pay for registration out of your better than expected profits.

Or use Click & Send. For satchels (and probably boxes) it's cheaper with signature required ($1) than a standard satchel without.