I fit a large king with polyester batting and flannel backing into a large flat rate USPS box.
I used a heavy duty garbage bag, put the bag in the box and folded the quilt so I could get it into the box (it stuck way up) then used my vacuum and sucked the air out of the bag until it was sucked down so I could close the lid, then tied off the bag and taped the package shut.
Since I only needed to do this for shipping, I didn't need a bag that would hold the seal for a log time. And I also needed it to fit a specific size box. It made it to where it was going fine, and poofed right up after being removed.