To broadly summarise the breakdown of the fee:
The £8 is the customs clearance fee that RM are charging you for acting as your customs agent.
It is for partly for presenting your package to customs. RM have to provide the invoice details to customs. They have to provide the package for...
This.
Fee2Pay covers underpaid items (both domestically and internationally), as well as customs charges which have been imposed by the UK Border Force (or whatever they're called these days).