Articulate way of speaking is a plus in post (7,8)

I believe the answer is:

express delivery

'a plus in post' is the definition.
(express delivery is a faster postal service)

'articulate way of speaking' is the wordplay.
'articulate' becomes 'express' (I've seen this before).
'way of speaking' becomes 'delivery'.
'express'+'delivery'='EXPRESS DELIVERY'

'is' is the link.

I've seen this clue in The Guardian.

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