You absolutely must use a very firm cardboard backing, and the pattern itself in a close fitting plastic bag (acid free is best.) I ship my delicate 40s - 60s patterns this way AND inside a box.
A cereal box backing will not work - the postal workers in my area have totally destroyed several vintage patterns and other vintage paper items I have bought that were packaged this way. And when I say totally destroyed, I mean that they were unusable, torn, and the delicate parts of paper reduced to dust and shreds. This is extremely disappointing to see valuable pieces of history in this condition, especially since they probably were not in this condition prior to shipping.
I know that other people who have posted here may find this a little extreme. However, the MAJORITY of delicate vintage patterns and vintage paper goods I have bought (eBay more so than here) have been damaged and close to unusable, and most of them did have cardboard backing. I did not complain, but I was very sad to see them in such poor shape. The seller had made an effort, but it was not enough with the way the postal service treats packages.
I wish that everyone would package vintage patterns much more carefully. It is very sad to see them damaged. Keep in mind that you never know what conditions they will go through (water, rapid humidity changes, postal machines with sharp edges and machines that pound on them, very rough postal workers, heavy packages sitting on them and bending them, etc.)
Thanks everyone for reading.
-Kelina