I wish to use a priority box instead of the priority envelop as the USPO cardboard priority envelops are not padded I think and don't show padded on the order page. So, I see 2 fit in a medium box but am unsure if 1 will fit in the small priority box or if you'd rather not pack it in a small priority box in which case I'll go with a medium. So....
Blades? If so, 1 will fit into a PADDED priority flat rate envelope. It's definitely there among the choices.
Up to 3 blades WILL fit into Region A box too.