I'm already paying HMRC for their time to get it right and tell me what I owe,
No you're not. When do you do this?
along with £8 to the Royal Fail for the priviledge of holding the parcel while HMRC does it's thing.
You're paying Royal Mail £8 to make the customs entry on your behalf.
Why would I then want to pay a retail supply chain extra money for doing what's already been done?
Because you're not paying someone else to do it.
For reference: I used to be an import broker for an international courier. HMRC has a system that commercial parties make declarations into. The idea that HMRC themselves deal with customs directly is rather old fashioned. I don't mean old fashioned as an insult, HMRC really did use to do all this back in the 60s/70s, however, across all taxes this has gradually changed to the taxpayer self-assessing and HMRC checking that assessment.