No, I haven't been double boxing. I cannot get 12+ eggs properly packed and upright in a #4 priority box (especially considering it's not unusual for me to pack 15-18 eggs or even more for a 1 dozen order).
However, I noticed the other day at the USPS store that the medium flat rate box is a little smaller than the #7, so I ordered a bundle and got them a couple days ago. I checked it out and they will fit inside the #7, so I'm going to try doubling boxing that way.
I just ordered a ton of different sized ones. I figure I will do a trial with some of them. I even got the shoebox size. I am wondering if 2 of those will fit in the large flat rate priority box????