I make 90% or more of my trips to the post office by bike. The smaller boxes are easy enough to fit in the saddle bag or panniers. The big ones, think surf boards and bikes, end up on the xtracycle for drop-off to UPS and FedEx. The medium ones are the challenge.
Today I had a vintage rack in a light but bulky box and improvised with some cord. It worked fantastic and had me thinking of some rack ideas John Cutter had designed a couple of years back. How are you getting the medium sized loads around by bike?