OP, this is the only potential exception in this case:
(5)The fourth exception is where—
(a)the vehicle is being used for the purposes of delivering goods to, or collecting goods from, any premises, or is being loaded from or unloaded to any premises,
(b)the delivery, collection, loading or unloading cannot reasonably be carried out in relation to those premises without the vehicle being parked as mentioned in subsection (1), and
(c)the vehicle is so parked for no longer than is necessary and for no more than 20 minutes.
My emphasis.
You are looking to qualify under (b), but we don't know anything about details, and details matter.
What were you taking to the Post Office?