Sunrise450- I just mash it down into a small size in a plastic bag and put it in the smallest envelope or box it will fit in. I include my name address and phone number inside the box and include my specifications on the design, batting, binding and backing, everything the website asks for. Several times I told her that backing and binding fabrics were her choice and have been absolutely thrilled at what she's come up with. She runs a brick and mortar quilt store, so she has fabric choices. It's always been fun getting them back. Sometimes its been a tone on tone backing, batik, or just muslin for a wall hanging.
I usually call in advance to start the conversation and verify the instructions. She will compress the quilt and send it back in the smallest box she can fit it in.
When I went to Hamilton Missouri to see MSQC, I visited Happy Crafters. They are exactly one hour south. It's a tiny store, but the people are very friendly.