This is the one I was thinking of:
https://store.usps.com/store/browse/...S_PriorityMail
Sorry that's so big. I ended up using a regular bubble envelope I had on hand.. but I would think this is big enough. I've gotten entire quilts sent to me in these packages before (twin size). But maybe the person mailing it was just a mail ninja.
Upon looking a little further, you can ORDER them ON the site... FOR FREE!!! I might just have to do that to cover all the upcoming rows I'll be shipping on the next 4 rounds.