For me I judge by what I'm shipping and how heavy.
If it's more than 1 or 2 pounds and will fit into a flat rate box I will use the flat rate box. Sometimes stuff just doesn't fit in flat rate box though.
Live plants I always ship priority and for those I generally use blank boxes cos bare root plants and cuttings are lightweight and oftentimes weird sizes that won't fit a flat rate box.
Useful information with all the plants shipping that occurs between forum members. I was worndering were some people get those " trianglar" mailers.
I've got a couple of plants that I was told were shipped to me almost 2 weeks ago; no tracking info, they are MIA - I do not have very much hope for a successful arival, the weather has taken a dive in Central MO