I think Matthew was copying my capitalisation of Post Office. Because it's still a brand name (and there is no competition), I always capitalise it. "I'm going to the Post Office" is the same as "I'm going to Waterstone's" or "I'm off to Sainsbury's". I agree that a "post office" is a type of business but, for me, it's a company name.
I would use "as soon as" as in post #2 or nothing at all: "when the Post Office opens". If you use nothing, "when the post office opens" has a meaning different from "when the Post Office is open".
Yes, that was the point I was going to make once we'd established how natural the rest of it was in AmE.
If the Post Office is open from 9am until 5pm, "Take it when/as soon as the Post Office opens" means "Take it at 9am" but "Take it when the Post Office is open" means "Take it any time between 9am and 5pm".