When I send mounted artwork I generally start with tissue, bubble wrap, cardboard back and front, add a bit of tape to keep them together, wrap in plain paper or large envelope, I then like to use the plastic mail bags, I get mine from the likes of Wilco but you can get them from the PO (they’re cheaper in Wilco), If I don’t have the large bags I have been known to use garden bin bags, they are thicker and a lot stronger than regular bin bags (mine are blue) I tape up with packing tape. I like the plastic outer covering because I think it’s stronger than paper and if it gets dropped in the rain it won’t soak through.
A quick note on the card although you probably know… If I use corrugated card I put the top and bottom the opposite way, so the ‘lines’ are not running the same way, if I think it needs it I will use a second layer of card.
I usually re-purpose card that i collect from all manner of shops, homebase, b&q etc as well as supermarkets are great places to get clean strong boxes.
Ooops I didn’t mean this to turn into novella, but you did ask, and I’m in a chatty mood tonight… BTW your art prints are lovely