No soil is the rule. Not sure abo8ut a sponge plug. i would have to read the regs again.I do not remember them giving an alternative. I have no problem with the things I ship because I ship Priority and the way I pack them they are good to go and get there in two days. Many of my customers are from CA and WA as well as TX and all we do is coordinate the heat or cold weather and the customers try and be home on the day they are expected. Works out so well I have yet to have a bad ship West. Now, NY is the worst. They must run over things with trucks before they deliver them. Twice I have had plants arrive out of the pots, etc. but they all lived and the customer and I got a laugh out of it because of how I had it packed. It was almost impossible to have it messed up but they managed to do it. The ones going to NYC seem to make it safely, but anyplace outside of the city.........look out. Hey you NYeers what goes with your Post Offices?????