I ship all of my quilts to QuiltsForKids and St. Judes in Ziploc 2 1/2 gallon bags. Their kits come in ziploc bags and I find that using the 2 1/2 gallon bags is a perfect fit. I zip the bag nearly all the way closed, leaving about an inch open then I roll it tightly to express the air and while tightly rolled, zip it closed. Doing it this way I am able to get 2 quilts in the large (12x12x5 1/2)Flat Rate USPS box. There has never been a question of condensation in my experience so far.
I wonder if my expressing most of the air is the difference. I might try a smaller (waste basket size) thin plastic bag sometime if I need to ship just one and the added bulk due to air isn't a problem.
My versios of the SpaceBag I guess. : = )