[mothered]

I thought that the courier would be involved seeing as sometimes they open an investigation and that will be registered with the courier.

Yes, but It's the company who's evaluating your claim so essentially, they have the final say of the events that have taken place pertaining to your account.

I’ve done a DNA before I found this forum and the courier come by insisting that he delivered the parcel

That's typical of the carrier driver taking matters Into his own hands, as opposed to being part of an Investigation.