OK, one more post. If I'm getting this right, you want to use the space bags, to shrink your quilt, to take advantage of the Postal Services Flat Rate Boxes. The main problem I see with your logic, while it sounds spot on, is that when you shrink the quilt, it becomes very stiff. Given that stiffness, unless the quilt is very small, you will have a difficult time maneuvering the quilt into the box. Logically it would work great, but in normal life, not so much. BUT!!!!!! try and see how it works. My visual may be different from your actual.
If I have it wrong, never mind!!
LOL