Isn´t this the post offices responsibility?
Bag with pocket at the back permits to conceal money/documents or iphone, You could also stuff it up till no more, at the front much less so.
Buyer still has no bag and probably paid $20 posting fees. It´s a difficult situation, but in my western understanding the business should cover for this (or have a respective insurance for lost packages and damaged items). What would be the situation if buyer shipped but item never appeared?
Myself I would pay the $50 if asked to and keep quiet - but never order again.