One point I have to disagree with Weevil on is who's compo claim it is.

You asked Parcel Force to provide a service and agreed a fee with them for doing so. That is the contract, so it is up to you to refund him and pursue a claim from the carrier. The buyer has no contract with Parcel Force, only with you.

Having said that, I claimed off them for an amp I bought and they paid out to me, not to the seller.