I hate to say this. I wouldn't and don't trust USPS with anything. Private companies do a much better job than government run organizations.
Case in point - I have had multiple packages delivered to my condo building which never get marked as delivered by USPS, yet I have them in my hand. If I were unscrupulous, I would contact the seller and say "never received, refund please".
My own personal code of ethics with this kind of thing: I am the seller and it is my responsibility to get the package into the buyers hands. If it gets lost then that is my fault and I owe them (unless you agreed anything else different up front).
But really glad it's in the UK now! Hopefully this one gets resolved positively... I had one like this where the package disappeared, I got a refund and it turned up 6 months later!