For shipping RIFs use Parcelforce, specifically express48 because it's the only one that doesn't prohibit low powered air weapons. You've also got to post it in the actual post office.
Black cling wrap (palette wrap) is a God send. You can get it pretty cheap for a decent sized roll on Amazon. I tend to fill gaps will brown parcel paper because it tends to be cheaper than bubble wrap but either works.
Once you’re happy with how secure everything is, finish off with a few passes of palette wrap to keep everything cinched in while also safely covering up box markings/images.