What you can do is ask them to write on the pack that customs see
'Value: £18'. It's highly unlikely customs would open the package.

...but if they do open it and find that the value of the goods is
much in excess of the declared value you'll be getting a
metaphorical 3am knock on the door. They don't take kindly to this
ruse :-(

And when they do, how many customs officials know that two 100uf 500v
Black Gate Capacitor have a value of £100 instead of £18 ?


Invoice included in the package?