I would fold it on the diagonal like this:
https://www.youtube.com/watch?v=y-waYZSCyOA, then fold back the corner so the label is right on top when she opens it. Write your notations about the fabric and design inside a card, and tuck or pin the card under the fold so she sees that, also. Can you ship it to someone who will be attending the shower, so they can open the box and ensure the presentation of the quilt is as you want it? (Instead of opening the wrong end of the box, for instance)
When you ship the quilt, seal it in a giant ziplock-style bag. You can get 2 gallon ziplocks at restaurant supply stores, or if that's too small, use one of those vacuum storage bags.
This part is
very important - make sure you put another label with her shipping address and your return address
inside the bag, where it's visible. Sometimes the shipping box gets wet (and not just from rain!) or damaged and the shipper has to re-box it. My son works for UPS. Please take my advice.