I ended up using priority mail, too. I've had bad experiences with packages languishing in post offices waiting to make it to the carrier. That doesn't happen when you have a higher level of service. I paid just over ten dollars for two day priority; that gave me four days' cushion on the due date of my project, just in case. I had to use a fairly big box for four tied quilts; if people made the fleece kind, they might get away with a flat rate box.
Sure enough, my blankets were received on a Wednesday, they were due the next Monday, and I printed my vouchers the Friday of that week. Now I just have to sit and count the days until June!