I have a box that is about the same size as a medium flat rate. Once I get to the post office (next week!), I'll have to see if it'll be cheaper to use the flat rate box or my box.
That's what I was going to do, too. Though I'm pretty sure flat rate boxes are only cheaper if it's super heavy, or if it's going past the Mississippi. Have fun on your snow day! I'm excited about crafting tomorrow too!