There's always more than one way of doing anything and all I can tell you is what I do. Perhaps other people will have other suggestions.
I only ever post unframed prints. Not even mounted - though plenty of other people post them with a mount. And I always post flat. My work is on heavy weight paper, usually with quite a lots of embossing, so rolling the paper and posting in tubes is not an option for me.
First I wrap the print in a little acid free tissue (something I get though my workshop but could also get from somewhere like The Great Art catalogue). Then I place between 2 pieces of card cut to size (something I always have plenty of and use to support unframed work in browsers at exhibitions ect) held together with little snips of cellotape. Add a business card, a discount code card and a postcard, and wrap the whole lot up in cellophane to water proof. (Large rolls obtained from Isca Bags on Marsh Barton - I see you are in Exeter). Lastly wrap in brown paper, label and post.
I hardly ever need to purchase brown paper because it comes in useful quantities every time I get more frames made.
This all suits me because it all materials I have around for various reasons. None of my work fits standard A4 or A3 packaging anyway so I'm better off using what I have round the place. I don't buy in any special packaging - other than little Moo stickers to hold the tissue closed. These are part of my branding.
