You can drop them in a red box however the only thing that will show on the tracking page is that you've paid for the package and when it is delivered.

I always lodge mine over the counter and obtain a receipt; that way the proof that you have posted it is available online to the buyer and will be accepted by PayPal as proof of posting.

I've heard some real horror stories about Post Offices, but never have any trouble with mine.  Each time new owners take over I train them;\